The invention relates to a control method for harmful gas in an on-orbit flying manned spacecraft cabin. The control method includes a harmful gas concentration control method in a normal mode and a harmful gas concentration control method in an emergency mode. Harmful gas concentration control in the normal mode is realized through a purification device, and a regeneration or non-regeneration method is used for purification. Harmful gas concentration control in the emergency mode is carried out through pressure relief and recovery. If the harmful gas concentration in the sealed cabin is beyond the purification capacity of a harmful gas control system, an astronaut can be temporally transferred to another cabin section or another docking manned spacecraft, pressure in the sealed cabin is relieved to the environment outside the cabin through a pressure relief device firstly, and then pressure recovery is carried out through a gas cylinder. After the pressure relief and recovery process is finished, the harmful gas concentration in the sealed cabin decreases to the lowest extent, and the requirement of harmful gas concentration control is met.