Abstract: Fault diagnosis is paramount to ensure the safe operation of reciprocating drilling pumps. This study focuses on a horizontally positioned three-cylinder single-acting drilling pump utilized at a drilling site. Experimental designs were implemented to collect and analyze hydraulic-end fault data. A pump frequency harmonic amplitude indicator is introduced by analyzing frequency components. The pump frequency is extracted to adapt to varying conditions, and harmonic amplitudes are derived using octave analysis. Comparisons between normal and faulty data enable the fault diagnosis, and the results are contrasted with four statistical indicators including root mean square, crest factor, margin factor, and kurtosis. With the pump frequency harmonic amplitude indicator, the diagnostic accuracy for valve body puncture faults is 93.9%, and 86.6% for spring decay faults, demonstrating significant potential in online fault diagnosis for reciprocating drilling pumps.