diff --git a/README_CN.md b/README_CN.md
index 158f2e4..ecb2cb2 100644
--- a/README_CN.md
+++ b/README_CN.md
@@ -176,6 +176,21 @@ module.exports = {
     }
   }
 }
+```
+
+ `viewportWidth` as function
+```js
+module.exports = {
+  plugins: {
+    // ...
+    'postcss-px-to-viewport': {
+      // options
+      viewportWidth({ file }) {
+        return file.indexOf('vant') !== -1 ? 375 : 750
+      },
+    }
+  }
+}
 ```
 
 #### 直接在gulp中使用，添加gulp-postcss
diff --git a/index.js b/index.js
index 6419322..1dfb2f0 100755
--- a/index.js
+++ b/index.js
@@ -118,7 +118,10 @@ module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
           size = opts.landscapeWidth;
         } else {
           unit = getUnit(decl.prop, opts);
-          size = opts.viewportWidth;
+          size = typeof opts.viewportWidth === "function"
+            ? opts.viewportWidth(css.source.input)
+            : opts.viewportWidth;
+            
         }
 
         var value = decl.value.replace(pxRegex, createPxReplace(opts, unit, size));