The invention provides a method for knowing the state of a main filter module of a water purifying device and the water purifying device. The water purifying device comprises a main flow circuit, a booster pump, a main filter module, a one-way valve, a pressure barrel, a pressure sensor and an analytical processing module, wherein a pressure-water storage volume curve of the pressure barrel is built in the analytical processing module; the analytical processing module is used for calculating the water storage volume of the pressure barrel according to pressure data fed back by the pressure sensor, and calculating water purifying speed of the main filter module based on the change rate of the water storage volume of the pressure barrel during a main filter module water producing period. According to the method for knowing the state of the main filter module of the water purifying device and the water purifying device provided by the embodiment of the invention, a customer can be reminded to replace a filter element in time.