The invention provides a control method and control system of an air conditioner and the air conditioner. The air conditioner comprises a compressor and a hot air bypass valve, and the hot air bypassvalve is connected with the compressor. The control method comprises the following steps that when operating frequency of the compressor reaches a preset frequency, the hot air bypass valve is opened;whether the hot air bypass valve reaches a closing condition or not is judged in real time; when the hot air bypass valve reaches the closing condition, the hot air bypass valve is controlled to be closed; and when the hot air bypass valve does not reach the closing condition, the hot air bypass valve is continuously controlled to be in an open state. According to the control method of the air conditioner, the opening and closing of the hot air bypass valve are controlled so that in a heating starting process of the air conditioner, low pressure of the system can be kept within a reasonable range, the situations that the low pressure of the system is low and more liquid is carried in an air return process due to the fact that the environment for starting the air conditioner is too severeor high-drop distribution pipes are used in an air conditioner installation process are avoided, and therefore running stability and reliability of the air conditioner are guaranteed.