The present invention discloses an asymmetrical PN junction thermoelectric couple structure and its parameter determination method. By changing the structural parameters of p-type semiconductor or n-type semiconductor, the current generated by p-type semiconductor is equal to the current generated by the n-type semiconductor, so that the high-efficiency output of PN junction thermoelectric couple can be realized. Meanwhile, the present invention provides a method for determining the parameters of PN junction based on the numerical solution method. Finally, the optimal size parameters of PN junction are obtained.