diff --git a/README.md b/README.md
index e161a8b..388521c 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,8 @@
     - pkgurl         字符串, 软件主页/上游地址
     - license        数组, 软件许可证
     - sources        数组, 构建过程使用到的所有源码包和补丁
+    - urls           数组, `sources` 各项对应的地址, 当前仅限使用 `wget` 获取
+    - md5sums        数组, `sources` 各项对应的 md5 checksum
     - options        数组, 构建选项, 详见第4小节
     - srcdir         字符串, 构建路径(在leaf主程序中定义)
     - pkgdir         字符串, 虚拟根目录(在leaf主程序中定义)
diff --git a/leaf b/leaf
index e546628..3049870 100755
--- a/leaf
+++ b/leaf
@@ -300,6 +300,35 @@ leaf_update_environment() {
 }
 
 leaf_prepare_package() {
+  local distdir url sourcefile index _md5sum
+  distdir="${DIST_DIR}/${pkgname}-${pkgver}"
+  mkdir -pv "${distdir}"
+  pushd "${distdir}"
+    for index in "${!sources[@]}"; do
+      sourcefile="${sources[$index]}"
+      url="${urls[$index]}"
+
+      if [ -f "${sourcefile}" ]; then
+        echo "${sourcefile} exits, checking..."
+        _md5sum=$(md5sum ${sourcefile} | awk '{print $1}')
+        if [ "${_md5sum}" == "${md5sums[$index]}" ]; then
+          echo "md5sum match, skip ${sourcefile}."
+        else
+          echo "md5sum does not match, redownloading..."
+          rm -rf "${sourcefile}"
+          wget "${urls[$index]}"
+        fi
+      else
+        echo "Downloading ${sourcefile}..."
+          wget "${urls[$index]}"
+        fi
+      _md5sum=$(md5sum "${sourcefile}" | awk '{print $1}')
+      if [ "${_md5sum}" != "${md5sums[$index]}" ]; then
+        leaf_error "md5sums of ${sourcefile}(${_md5sum}) does not match with ${md5sums[$index]}!"
+      fi
+    done
+  popd
+  echo "Sources ready."
   src_prepare
 }