The utility model relates to a balance valve assembling device for a cold and hot water mixing valve with improved structure. The utility model is characterized in that a concave circular lower seat pit for a balance valve is arranged above a cold and a hot water inlet holes of the foot wall of the box body of a mixing valve, an inwardly dished circular upper seat pit for a balance valve is arranged on the position of the cold and the hot water inlet holes of the lower surface of a cover plate of the balance valve, and a sealed pit is arranged on the periphery of a water outlet hole and the upper seat pit for a balance valve. By the compression of the sealed pit of the balance valve and the footwall of the box body, the device causes the lower seat pit for a balance valve of the box body and the upper seat pit for a balance valve of the cover plate to form leak-stopping space together, and causes the balance valve to be fixed and sealed in the leak-stopping space. Each component can be pinpointed in assembling, so the efficiency of easy operation and speedy assembling can be achieved.