the main reason maybe QCA9880 is not compatible with intel wireless card (my test pc config). if "Throughput Booster" is enabled, the initial wifi connection will break soon under heavy data flow, sometime cause the router reboot. in version 6.44.3, I found the router always reboot with heavy data flow. I downgrade to 6.39.x and 6.38.x and 6.34.x, no reboot again, just connection break.
in the meantime, I also found if "Throughput Booster" is enabled, the router may cost much more power than usual, and that may cause a short time power failure. that maybe the reason why router reboots.
After I turn off the "Throughput Booster", everything looks ok now. router is running 6.34.x, I will try to upgrade it to 6.44.3 later and check if it stable.
before this trouble, "Throughput Booster" is enabled for a long time and running older routeros version, and I don't aware of any router crash, except the initial connection a little unstable.
thank you everyone helps me!