The invention discloses a serial port expansion device for an embedded microprocessor. The serial port expansion device comprises the microprocessor and a programmable logic device. The microprocessor comprises M serial ports and N control ports. The programmable logic device comprises K universal input and output ports, wherein K>M>0, and N>0. The microprocessor is used for setting port communication modes of the programmable logic device through the N control ports. The programmable logic device is used for programming a programmable logic circuit inside the device and controlling the communication interconnection relation of the universal input and output ports on the programmable logic device. The universal input and output ports of the programmable logic device are connected with one or more peripheral devices, and signal transmission between the peripheral devices and the microprocessor and/or signal transmission between the peripheral devices are achieved. According to the serial port expansion device of the embedded microprocessor, the number of the ports is convenient and easy to increase, switching is flexible, cost is low, efficiency is high, and transportability is high.