In order to enhance Electromagnetic Compatibility (EMC) of monitoring system, the EMI distribution should be obtained. But the measurement could not be accomplished perfectly because of underground condition. A novel EMI soft-sensing method was proposed to measure the electromagnetic interference(EMI) indirectly. The wavelet time-frequency analysis was introduced to monitoring system and the EMI source could be ascertained. The startup time, duration and waveform could be detected based on the running records of underground electric equipments. The electrical fast transient/burst (EFT/B), the most outstanding interference, was studied to verify the validity of wavelet analysis. EMI filter was improved in accordance of the EMI gotten from wavelet analysis. Power port immunity was developed obviously. Therefore the EFT/B of data port was restrained markedly with wavelet filtration. Coordinative effect of EMI power and wavelet filter resulted in reduction of false alarms. Results indicate that wavelet analysis and the improved EMI filter have enhanced the EMC of monitoring substations and sensors obviously. It is concluded the wavelet analysis is of guiding significance to measure underground EMI distribution and EMC of the underground monitoring system.