This paper presents a partially resonant active filter based on a digital PWM control circuit with a DSP that can improve the power factor and input current harmonic distortion factor of distributed power supply systems in communications buildings. The steady-state and dynamic characteristics of this active filter are analyzed experimentally and the relationship between the control variables of digital control circuit with the DSP and performance characteristics such as regulation of the output voltage, input power factor, input current harmonic distortion factor, boundaries of stabilities and transient response are defined. Using the partially resonant circuit, the efficiency is over 91%, which is 0.9 point higher than that of non-resonant circuit and the high frequency switching noise is suppressed. Furthermore, the digital control strategy with the DSP proposed in this paper can realize the superior transient response of input current and output voltage for the step change of load, the power factor over 0.99 and total harmonic distortion factor less than 1.1%.