The invention provides a main transformer online regeneration respirator with double microwave heating devices and a using method of the main transformer online regeneration respirator. The main transformer online regeneration respirator comprises an upper microwave heating device, a lower microwave heating device, a first moisture absorption tank, a second moisture absorption tank and a three-way pipe; the lower end of a respiration pipe of a main transformer expansion tank is respectively communicated with the first moisture absorption tank and the second moisture absorption tank through the three-way pipe; a second port and a third port of the three-way pipe are provided with a control valve; the upper microwave heating device is respectively connected with the upper parts of the first moisture absorption tank and the second moisture absorption tank through two waveguide pipes; the lower microwave heating device is respectively connected with the lower parts of the first moisture absorption tank and the second moisture absorption tank through two waveguide pipes. According to the main transformer online regeneration respirator, the problems that major potential safety dangers exist since a traditional main transformer large-size respirator needs to be manually replaced when a moisture absorbent loses efficiency after saturation by moisture absorption and a main transformer heavy gas protection device needs to be stopped in the replacement process are solved.