The invention provides a mud circulation system for reducing swab pressure while tripping out, including drilling tool components, a normal drilling circulation channel and a tripping circulation channel, the drilling tool components include a drill string, a drill bit, and a top drive, the normal drilling circulation channel includes a first rotary valve, a solid phase control device, a mud tank, a mud pump, and a forth rotary valve connected in sequence, the tripping circulation channel includes a second rotary valve, a tripping mud pump, a tripping mud tank and a third rotary valve connected in sequence; the mud circulation system for reducing the swab pressure is operated to: shut down the mud pump, close the first rotary valve, and open the second rotary valve and the third rotary valve before tripping out; determine the pumping flow rate according to relevant parameters.