文档中有一个:row-class-name="tableRowClassName"属性，可以获取到当前行的index

<template>
  <el-table
    :data="tableData2"
    style="width: 100%"
    :row-class-name="tableRowClassName">
    <el-table-column
      prop="date"
      label="日期"
      width="180">
    </el-table-column>
    <el-table-column
      prop="name"
      label="姓名"
      width="180">
    </el-table-column>
  </el-table>
</template>

<style>
  .el-table .warning-row {
    background: oldlace;
  }
  .el-table .success-row {
    background: #f0f9eb;
  }
</style>

<script>
  export default {
    methods: {
      tableRowClassName({row, rowIndex}) {
        if (rowIndex === 1) {
          return 'warning-row';
        } else if (rowIndex === 3) {
          return 'success-row';
        }
        return '';
      }
    },
    data() {
      return {
        tableData2: [{
          date: '2016-05-02',
          name: '王小虎'
        }, {
          date: '2016-05-04',
          name: '王小虎'
        }]
      }
    }
  }
</script>

只要将方法，构造成这样：把每一行的索引放进row

tableRowClassName ({row, rowIndex}) {
      // 把每一行的索引放进row
      row.index = rowIndex;
}

构造row的index，后续用到时只需要row.index获取索引，从0开始