when i lock the phone my wifi disconnect why?

It’s a feature that has been implemented in all iPhones since the first to preserve battery life. Wi-Fi uses power continuously when it is connected to a Wi-Fi network, whether it is exchanging data or not. If Wi-Fi stayed on continuously when the phone was asleep users would be very unhappy about their significantly reduced battery life. You can override this feature by leaving the phone connected to power, either line power of a battery pack.

Save battery power.

Leave the phone connected to the power adapter to keep Wi-Fi active.