The invention relates to a toilet flushing system, which comprises a negative pressure forming part, a flushing part and a toilet flushing part, wherein the toilet flushing part comprises a toilet flushing body, a drain pipe, an electromagnetic valve, a pressure sensor and a one-way valve; the flushing part is communicated with the toilet flushing body; the electromagnetic valve is arranged at thejoint of the toilet flushing body and the drain pipe; the one-way valve is arranged in the drain pipe; the pressure sensor is arranged in a cavity of the drain pipe between the electromagnetic valveand the one-way valve; the negative pressure forming part is communicated with the cavity of the drain pipe between the electromagnetic valve and the one-way valve; and the pressure sensor is connected with the electromagnetic valve in signal manner. The water-saving effect of the toilet flushing system is improved, and the toilet flushing system has the functions of an electromagnetic valve, a one-way valve and a negative pressure forming part, wherein the electromagnetic valve is firstly opened to discharge excrement into a pipeline, and then water flow flushing is carried out, so that odoris effectively prevented from overflowing.