Air carrier Captain reported momentary loss of aircraft control after encountering turbulence at FL380 while under control of ZSHA Control Center. Reportedly; the aircraft rapidly began to climb; the crew received an Altitude Alert before disconnecting the autopilot to manually regain control.

Narrative

With Shanghai Control approaching point SADLI; crew requested 5 NM Right due to weather seen on radar - was granted by Shanghai ATC (just transferred) from ICN Control. Aircraft was at FL380; autopilot ON. Did immediate right turn to avoid weather at approximately 2 NM right of course encountered moderate turbulence; Heads-Up Display (HUD) EVIS was blank; aircraft rapidly started to climb - 'Altitude Alert' came on at +300 ft.; I immediately turned off the autopilot and pulled back the throttles to get back to altitude; aircraft deviated +500 ft. before being corrected. Re-engaged autopilot at FL380 and the aircraft immediately pitched over; but this time I turned off the autopilot and corrected before -300 ft. During this time Shanghai Control saw our altitude deviation - First Officer (FO) said we were correcting as I had my hands full. The whole incident was about 15 seconds. After leveling again at FL380 - re-engaged autopilot thereafter with no incident. When aircraft under control; I radioed ATC and told them we had moderate turbulence in the weather causing our altitude deviation. No other transmission on the subject was noted. Descended and landed without incident.Cause: Weather turbulence; wind shear