1、配置文件

/app/code/Aiweline/Demo/etc/system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_SalesAgreement:etc/system_file.xsd">
    <system>
        <tab id="Aiweline" translate="label" sortOrder="10">
            <label>Aiweline_Demo标签名字随意</label>
        </tab>
        <section id="Aiweline_Demo_admin" translate="label" sortOrder="0" showInDefault="1"
                 showInWebsite="1"
                 showInStore="1">
            <class>separator-top</class>
            <label>Demo样本</label>
            <tab>Aiweline</tab><!--指定粘贴在哪个tab上，我们的tab是Aiweline-->
            <resource>Aiweline_Config::config</resource>
            <group id="general" showInDefault="1" showInStore="1" showInWebsite="1" sortOrder="1"
                   translate="label">
                <attribute type="expanded">1</attribute>
                <label>基本配置</label>
                <field id="DemoType" translate="label" type="select" sortOrder="1" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>下拉选择</label>
                    <source_model>Mjd\JiujiuApiyiliubabaProductImport\Model\Config\Source\DemoType</source_model>
                </field>
            </group>
        </section>
    </system>
</config>

2、类model

/app/code/Aiweline/Demo/Model/Config/Source/DemoType.php

<?php
/**
 * Created by PhpStorm.
 * User: 秋枫雁飞
 * Date: 2019/11/14
 * Time: 10:02
 * Web:www.aiweline.com
 * Copyright © 2016 Aiweline. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Mjd\JiujiuApiyiliubabaProductImport\Model\Config\Source;
/**
 * @api
 * @since 100.0.2
 */
class DemoType implements \Magento\Framework\Option\ArrayInterface
{
    /**
     * Options getter
     *
     * @return array
     */
    public function toOptionArray()
    {
        /* normal(综合),va_rmdarkgmv30rt(成交额),price-asc(价格从低到高),price-desc(价格从高到低)*/
        return [
            ['value' => '1', 'label' => __('demo 1')],
            ['value' => '2', 'label' => __('demo 2')],
            ['value' => '3', 'label' => __('demo 3')]
    }
    /**
     * Get options in "key-value" format
     *
     * @return array
     */
    public function toArray()
    {
        return [0 => 'demo1', 1 => 'demo2', 2=> 'demo3'];
    }
}

3、模块创建自行了解