The invention relates to a vehicle-mountedhead-up display system driving mechanism which comprises a fixing frame, a left support, a right support and a driving device arranged on the right support. A driving gear is arranged on the driving device, a semi-gear which can be in rotary meshed connection with the driving gear is arranged at the position, close to the driving device, on the right side of the fixing frame, and the fixing frame can be driven to turn over in the vertical direction through drive of the driving gear and the semi-gear. A spring is further detachably connected between the bottom end of the fixing frame and the right support, turnover of the fixing frame drives the spring to produce stretching shape change, and a drive gap between the driving gear and the semi-gear can be eliminated through stretching of the spring. The vehicle-mountedhead-up display system driving mechanism is simple in structure, low in cost and high in reliability, can achieve adjustment of an inclination angle of a reflecting mirror and can also ensure the stability of the reflecting mirror in all direction under shaky situation.