The invention discloses a control method of an air conditioner. The control method comprises the following steps: S1, detecting current indoor temperature, current outdoor temperature and current indoor relative humidity; S2, acquiring indoor temperature grade corresponding to the current indoor temperature and acquiring relation grade between the current indoor temperature and the current outdoor temperature; S3, determining a current operation mode of the air conditioner according to the indoor temperature grade and the current indoor relative humidity and relation grade. According to the method provided by the embodiment of the invention, operation of setting temperature by a user is not required for the air conditioner in the entire operation process of the air conditioner, so automatic switching in a refrigeration mode, a heating mode, an air supply mode, a dehumidification mode and the like of the air conditioner is realized, the operation steps of the user are simplified, the comfort is improved, and the user experience is improved. The invention also discloses a control device of the air conditioner and the air conditioner with the control device.