Capacitor voltage transformer (CVT) is widely applied to electric power system. It is beneficial to acquire the accurate transfer function of CVT for the protection and the monitoring of power system. At present, the reaserches about CVT are based on its typical equivalent circuit, however, due to the different internal parameters of CVT under different operating conditions, the transfer functions are different, which leads to inaccurate research results before. A square-wave pulse is applied to CVT when the internal structure parameters are not clear, then, the recursion method and the least square method are applied to caculate the impulse response of CVT based on convolution theorem, and Laplace transform is used to obtain the frequency transfer characteristics. The correctness of the theory and the reliability of the algorithm are verified by building the simulation circuit in PSCAD. Furthermore, the noise is added in the output signal, the anti-noise ability and noise impact factor of these two algorithms are analyzed in their algorithm structure. It is found that when noise is added, the anti-noise ability of the recursive method and the least square method are influenced by the length of convolution, both methods can get accurate results when the signal to noise ratio is larger than 40 dB.