The invention relates to a sub-synchronous resonance monitoring method. The method comprises the following steps: 1) acquiring line current in a series compensation system, and performing low-pass filtering on the line current; 2) sampling the line current after low-pass filtering, wherein the sampling indicates sampling one sampling value every M sampling values so as to convert sampling values with high sampling rate into sampling values with low sampling rate, and M is more than 1; 3) performing band-pass filtering on the sampled sampling values to generate sub-synchronous resonance current; and 4) calculating the effective value ISSR of the sub-synchronous resonance current, judging whether the ISSR is greater than the fixed value of the sub-synchronous resonance current, and if so, bypassing the series compensation system. The method eliminates circuit parameter conditions generated by sub-synchronous resonance, protects the safety of a generator shaft system, and improves the stability and the reliability of a system; and the measures of sectional calculation, sectional update, accumulation and the like can be taken by adopting sampling and a long-window effective value algorithm, so that the calculation quantity is small and relay protection can be realized.