给input添加:style="{width: '100%'}"即可
 <el-row :gutter="15">
        <el-form ref="seForm" :model="selectFormData" :rules="rules" size="medium" label-width="100px">
          <el-col :span="3">
            <el-form-item label-width="60px" label="年度：" prop="coopname">
              <el-date-picker
                v-model="selectFormData.year"
                type="year"
                placeholder="选择年"
                :style="{width: '100%'}">
              </el-date-picker>
            </el-form-item>
          </el-col>
          <el-col :span="5">
            <el-form-item label="合作社名称：" prop="coopname">
              <el-input v-model="selectFormData.coopname" placeholder="请输入合作社名称" clearable
                :style="{width: '100%'}"></el-input>
            </el-form-item>
          </el-col>
          <el-col :span="7">
            <el-form-item label="日期范围：" prop="timerange">
              <el-date-picker type="daterange" v-model="selectFormData.timerange" format="yyyy-MM-dd"
                value-format="yyyy-MM-dd" :style="{width: '100%'}" start-placeholder="开始日期"
                end-placeholder="结束日期" range-separator="至" clearable></el-date-picker>
            </el-form-item>
          </el-col>
          <el-col :span="3">
            <el-form-item>
              <el-button type="primary" @click="selectForm">查询</el-button>
            </el-form-item>
          </el-col>
        </el-form>
      </el-row>