The impact of intake humidity on heavy-duty natural gas engine performance and emission was studied based on the engine that meets China Ⅵ emission standard. The research results show that the intake humidity has a certain influence on the performance and emission of natural gas engine. With the increase of intake humidity, the combustion deteriorates, the in-cylinder combustion pressure decreases, and the CA50 moves backwards. The engine ignition angle needs to be advanced to ensure normal engine operation. The torque output decreases and the gas consumption increases. Besides, HC and CO emissions show an upward trend, but NO_x emission reduces with a large influence.