FDTD is an electromagnetic field computation method with the ability of considering circuit elements. Traditional lump element method is insufficient for simulating circuit. In this thesis, we use equivalent source method to combine non-linear circuit elements like active devices into the FDTD simulation. The advantages of this is powerful and time-saving. The accuracy of this method is checked of transmission line driving by CMOS circuits. By employing this method, we find that it will increase EMI phenomenon by strengthening current of driving load, and the load of coupling line will affect noise due to impedance mismatch.