The invention relates to a temperature and humidity control device for a low-temperature and constant-temperature and humidity box and a method. The temperature and humidity control device includes a constant-temperature and humidity external control unit, a constant-temperature and humidity internal control unit and an airflow channel therebetween; a heater, an evaporator and a turbulence heat exchange excitation fan are arranged in the airflow channel; the constant-temperature and humidity internal control unit is a closed constant-temperature and humidity unit comprising a test piece door with a visual window; a turbulence humidification fan and a humidification water tank are arranged at the bottom of the constant-temperature and humidity unit; the upper cover of the humidifier water tank is a louver which can be closed; the upper part of the constant-temperature and humidity internal control unit is provided with an adjustable heat and humid exchange opening which makes the interior and exterior of the constant-temperature and humidity internal control unit are communicated controllably; and the constant-temperature and humidity internal control unit is provided with a temperature and humidity test probe and a test piece tray. The temperature and humidity control device of the invention is characterized in that the constant-temperature and humidity unit operates independently and is not directly in cross linkage with heating and cooling units; and the constant-temperature and humidity unit can be in controllable linkage with the outside so as to carry out humid elimination. According to an operation method, at first, temperature is adjusted, and then independent humidification is carried out; and if humidity is too large, the adjustable heat and humid exchange opening is opened so as to be communicated with the outside to carry out humid elimination.