An approach to measuring the frequency response of high-speed photodiodes (PDs) is proposed and experimentally demonstrated based on employing an ultrashort optical pulse train to sample an envelope-modulated microwave subcarrier. Through up-and-down conversion sampling, a varying frequency component and a fixed low frequency component can be obtained, where the varying frequency component probes the ultra-wideband response information of PD. Through measuring the relative amplitude between the two frequency components, the frequency response of the PD at the any frequency within ultra-wideband frequency range can be calculated by taking the response at the fixed low frequency component as a reference. Thereinto, the frequency response of the electro-optic modulator is cancelled out, and the uneven comb intensity introduced by the ultrashort optical pulse train can be corrected by choosing the specific frequency of the microwave subcarrier. In the proof-of-concept experiment, the self-calibrated frequency response measurement of a commercial PD is demonstrated by employing an optical pulse train with a repetition rate of 9.954 GHz and an electro-optic frequency sweeping up to 4.977 GHz. The frequency measurement range is achieved up to 49.77 GHz, and the frequency resolution reaches 300 kHz in the rough measurement and 10 Hz level in the fine measurement. The consistency between the proposed method and conventional methods proves the ultra-wideband and hyperfine frequency response measurement of PDs.