The invention discloses a gas engine operation method under the low gas pressure condition. The method comprises following steps that an engine runs; according to the rotating speed of the engine, the maximum injection pulse width allowable under the current condition, driving torque needed by a driver, the maximum gas flow allowable under the current condition and the maximum air inlet amount A allowable under the current condition are calculated according to the rotating speed of the engine; according to the calculated driving torque needed by the driver, an air inlet amount B is obtained through calculation; the maximum air inlet amount A allowable under the current condition and the air inlet amount B are compared, and through the small value of the maximum air inlet amount A and the air inlet amount B, an air inlet amount C is obtained; and according to the air inlet amount C, a needed gas flow and a needed injection pulse width are calculated. When the current gas pressure is low, through the slow control over the air inlet amount, the torque output of the engine is slowly reduced; and through active adjusting, the air amount is reduced so as to reduce the torque output, the condition that the torque is suddenly reduced is prevented from appearing, vehicle flameout and shaking are avoided, and safe and stable driving is ensured.