When a square wave voltage u with the period T，the amplitude Um and the duty ratio δ is used as the excitation，it is deduced explicitly by a simple method that the values of the response，uc，the voltage across the capacitor，on the rising edge and falling edge will converge monotonically and continuously to Um(1-A1 )A2 / (1-A1 A2)and Um(1-A1 )/ (1-A1 A2 )，respectively (where A1 = e-Tδ / (RC)，A2 = e-T(1-δ)/ (RC)).Moreover，the relationships among the parameters are also discussed. The results are not only useful for understanding and analyzing the experimental phenomena of the first-order RC transient process under square wave excitation，but also helpful for improving the experimental design.