页面

<view class='container'>
  <image src='' class='bg-blur'></image>
  <view>content</view>
</view>

css

.bg-blur{
  position: fixed;
  top: 0;
  left: 0;
  border: 0;
  width: 100%;
  height: 100%;
  filter: blur(6px);
  z-index: -1;
}

效果