[Vuejs]-Why transition component not work in vue2?

Initially, I want to have the transition effect when this component mounted firstly.Just now, I read the vue2 document transition part in detail and find first mount transition part.So I amend my code like this:

<template>
  <transition name="page-one-load" appear>
   ...
  </transition>
</template>

Yes, I append ‘appear’ modifier to transition label, then it worked!