Abstract: The crack formation mechanism of hole of the 30CrNi3MoV fracturing pump fluid end valve box was investigated by macroscopic examination, fracture analysis, composition and inclusion examination, microstructure and crack propagation path observation. It was found that the chemical compositions meet requirements and the high cleanliness is confirmed for the valve box. The microstructure of valve box is tempered sorbite and grain size is ascertained to be No 8.0~8.5 grade. Crack initiation and propagation are independent of the compositions and microstructure of valve box. However, it was found that there are some micropores with a size of about 1 μm near the crack. The service life of valve box is stable at 300 h and the similar failure does not be occurred by decreasing power remelting and increasing removal rate of electroslag head and tail.