diff --git a/src/core/render/compiler.js b/src/core/render/compiler.js
index a13faa663..58db9b3a8 100644
--- a/src/core/render/compiler.js
+++ b/src/core/render/compiler.js
@@ -12,7 +12,7 @@ let markdownCompiler = marked
 let contentBase = ''
 let currentPath = ''
 let renderer = new marked.Renderer()
-const TOC = {}
+const cacheTree = {}
 let toc = []
 
 /**
@@ -120,11 +120,9 @@ export function sidebar (text, level) {
 export function subSidebar (el, level) {
   if (el) {
     toc[0] && toc[0].level === 1 && toc.shift()
-    const tree = genTree(TOC[currentPath] || toc, level)
+    const tree = cacheTree[currentPath] || genTree(toc, level)
     el.parentNode.innerHTML += treeTpl(tree, '<ul class="app-sub-sidebar">')
-  }
-  if (toc.length) {
-    TOC[currentPath] = toc.slice()
+    cacheTree[currentPath] = tree
   }
   toc = []
 }
diff --git a/src/core/render/gen-tree.js b/src/core/render/gen-tree.js
index 84ff05e78..d3f6fa238 100644
--- a/src/core/render/gen-tree.js
+++ b/src/core/render/gen-tree.js
@@ -15,8 +15,7 @@ export function genTree (toc, maxLevel) {
 
     if (level > maxLevel) return
     if (last[len]) {
-      last[len].children = last[len].children || []
-      last[len].children.push(headline)
+      last[len].children = (last[len].children || []).concat(headline)
     } else {
       headlines.push(headline)
     }