diff --git a/internal/accounts/picker.go b/internal/accounts/picker.go
new file mode 100644
index 0000000..3ee2806
--- /dev/null
+++ b/internal/accounts/picker.go
@@ -0,0 +1,56 @@
+package accounts
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/wangjia/pay/config"
+)
+
+// ErrNoAccount 表示该 channel/region 下没有可选账户(候选为空,或被策略全部排除)。
+var ErrNoAccount = errors.New("accounts: no candidate account for channel/region")
+
+// 路由策略名(配置 routing.<channel> 的取值;缺省/未知一律当 round_robin)。
+const (
+	StrategyRoundRobin = "round_robin"
+	StrategyWeighted   = "weighted"
+	StrategyLimitAware = "limit_aware"
+)
+
+// PickHint 选账户的上下文提示(确定性 + retry 换号)。
+type PickHint struct {
+	OutTradeNo      string   // weighted 的确定性锚点;crypto 地址池亲和
+	AmountMinor     int64    // limit_aware 判定 used+amount<=DailyLimit
+	ExcludeAccounts []string // retry:排除已试过的账户,换一个重试
+}
+
+// Picker 面向 gateway 的路由入口:按渠道配置的策略从 enabled 账户里选一个。
+type Picker interface {
+	Pick(channel, region string, hint PickHint) (config.AccountConfig, error)
+}
+
+// Strategy 是纯函数式选择器:对「已过滤的候选切片」选一个账户。
+// key = channel+"|"+region,round_robin 用它分桶进程计数器,其余策略忽略。
+type Strategy interface {
+	Pick(key string, candidates []config.AccountConfig, hint PickHint) (config.AccountConfig, error)
+}
+
+// RoundRobin 进程内轮询:按 key 分桶各自递增计数器,mod 候选数。
+// 重启归零、不落库、不跨实例共享(见计划 D2);只为均摊,不作强配额。
+type RoundRobin struct {
+	mu       sync.Mutex
+	counters map[string]uint64
+}
+
+func NewRoundRobin() *RoundRobin { return &RoundRobin{counters: map[string]uint64{}} }
+
+func (r *RoundRobin) Pick(key string, candidates []config.AccountConfig, _ PickHint) (config.AccountConfig, error) {
+	if len(candidates) == 0 {
+		return config.AccountConfig{}, ErrNoAccount
+	}
+	r.mu.Lock()
+	n := r.counters[key]
+	r.counters[key] = n + 1
+	r.mu.Unlock()
+	return candidates[int(n%uint64(len(candidates)))], nil
+}
diff --git a/internal/accounts/picker_test.go b/internal/accounts/picker_test.go
new file mode 100644
index 0000000..6cdccb5
--- /dev/null
+++ b/internal/accounts/picker_test.go
@@ -0,0 +1,47 @@
+package accounts_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/wangjia/pay/config"
+	"github.com/wangjia/pay/internal/accounts"
+)
+
+func cands(ids ...string) []config.AccountConfig {
+	out := make([]config.AccountConfig, 0, len(ids))
+	for _, id := range ids {
+		out = append(out, config.AccountConfig{AccountID: id, Channel: "fake", Region: "global", Enabled: true, Weight: 1})
+	}
+	return out
+}
+
+func TestRoundRobinCyclesInOrder(t *testing.T) {
+	rr := accounts.NewRoundRobin()
+	cs := cands("a1", "a2", "a3")
+	want := []string{"a1", "a2", "a3", "a1", "a2"}
+	for i, w := range want {
+		got, err := rr.Pick("fake|global", cs, accounts.PickHint{})
+		if err != nil || got.AccountID != w {
+			t.Fatalf("call %d: got %v err %v, want %s", i, got.AccountID, err, w)
+		}
+	}
+}
+
+func TestRoundRobinKeysAreIndependent(t *testing.T) {
+	rr := accounts.NewRoundRobin()
+	cs := cands("a1", "a2")
+	// 不同 key(渠道/区域)各自独立计数,互不串扰。
+	g1, _ := rr.Pick("alipay|cn", cs, accounts.PickHint{})
+	g2, _ := rr.Pick("wechat|cn", cs, accounts.PickHint{})
+	if g1.AccountID != "a1" || g2.AccountID != "a1" {
+		t.Fatalf("每个 key 首次都应从 a1 起, got %s / %s", g1.AccountID, g2.AccountID)
+	}
+}
+
+func TestStrategyEmptyCandidates(t *testing.T) {
+	rr := accounts.NewRoundRobin()
+	if _, err := rr.Pick("fake|global", nil, accounts.PickHint{}); !errors.Is(err, accounts.ErrNoAccount) {
+		t.Fatalf("空候选应 ErrNoAccount, got %v", err)
+	}
+}