When ordering food on McDonald's mobile phone, you can choose Diet Coke according to the following steps:

1. Open the subscription software.

2. Enter "McDonald's" in the search bar and click "Enter".

3. Select the goods you want in the menu bar.

4. When choosing a drink, find the "Coke" option and check "Sugar-free".

5. Confirm the order information and complete the payment, then you can get the required sugar-free cola.

The above steps are applicable to most McDonald's mobile phone ordering platforms, and the specific operation may be different due to different software or system versions.