Tap the WhatsApp app icon, which resembles a white speech bubble and phone receiver on a green background. This will open the WhatsApp main page if you're logged in. If you aren't logged in, follow the on-screen prompts to enter your phone number and select a username. This gear-shaped icon is in the bottom-right corner of the screen. Doing so opens the Settings menu.  If WhatsApp opens to a chat, first tap the "Back" button in the top-left corner of the screen. If WhatsApp opens to a page with the word "Settings" at the top, you're already in the Settings menu. It's next to a chat bubble icon in the middle of the page. This option is at the bottom of the page. Tap the "your phone number" text field in the middle of the page, then enter the phone number that you used to create your WhatsApp account. It's below the phone number text field. All the chats, including text and media, will be deleted from your iPhone or iPad. You may have to close and re-open WhatsApp before your iPhone's storage will reflect the free space from the deleted data.

TL;DR in Chinese:
打开WhatsApp。 点按设置。 点按对话。 点按页面底部的删除所有对话选项。 出现提示时，输入你的手机号码。 点按手机号码文本框下面的删除所有对话。