when electronic transducer is used for different applications， it is often necessary to convert the sampling data of different sampling frequency to that with a same sampling frequency if the bus protection device is connected to electronic current transformers（ECTs） of different rated sampling frequency. This paper presents the concept of time-domain continuous finite impulse response（TCFIR） filter. A continuous digital low pass filter based on TCFIR is proposed to acquire continuous signal from electronic potential transformer（EPT）/ECT discrete sampling data and to calculate instantaneous value at any moment. Simulation using 1 000 kV transmission line fault current with strong harmonics shows that the proposed sampling frequency conversion from 4 kHz to 2.4 kHz is of only a peak error less than 0.66%. Meanwhile， the data window length is 6 points and the time delay is only 0.75 ms. The accuracy and real-time characteristics of the algorithm meet the requirement of bus protection.