在网上查了很久，也试了各种解决办法，如下情况都无法解决问题：

@click="onSubmit"

@click.native="onSubmit"

:click="onSubmit".

最后用v-on:click="onSubmit"解决了问题，具体代码如下：

<el-form-item>
    <el-button type="primary" v-on:click="onSubmit" round>更新数据</el-button>                                                   
</el-form-item>

<script>
    var Main = {
        data() {
            return {
                tabPosition: 'left',
                form: {
                    //CIM_type: '',
                    //CIM_date: '' ,
                    type: '',
                    date: ''  
                }
            }
        },
        methods: {           
            onSubmit() {
                console.log(this.form.type)
                console.log(this.form.date)
                var D_api = "";
                this.$http.post(D_api, { params: { val: this.form.date} }).then(
                    function (response) {                        
                        if (response.data.code == 0) {
                            Toast({
                                content: '数据更新成功！',
                            });
                        }                       
                        else if (response.data.Code == 500) {
                            Dialog.failed({
                                title: '数据更新失败',
                                content: response.data.msg,
                                confirmText: '确定',
                                onConfirm: () => console.log('[Dialog.failed] confirm  
                                 clicked'),
                            });
                        }
                        else {
                        }
                    }
                ).catch(function (response) {
                    // console.log("400", response);
                });
                
            },
        }
    }
    var Ctor = Vue.extend(Main)
    new Ctor().$mount('#app')
</script>