The hydraulic characteristics of the variable primary flow system with on-off valves under temperature difference control is investigated, and the nonlinear heat exchange coupling property of fan coil units and rooms is combined to further explore the applicable conditions of temperature difference control and setting procedures of temperature difference value(TDV). The result indicates that the temperature difference control in the system with on-off valves at the terminal is also applicable when the pipe network's load is equally distributed and the load changing rule of each opening user is similar. In this case, the system's hydraulic disorder has smaller impact on the indoor temperature and humidity. When adopting variable temperature difference control, setting TDV piecewise according to different total load rates cannot have good control of indoor temperature and humidity. Therefore, it is recommended to set TDV according to ensure fan coil's dehumidification capacity and indoor dry-bulb temperature preferentially, compared with current methods, the stability of indoor temperature can be guaranteed under the same fluctuation of the indoor relative humidity.