The failure of cylinder head cracking involves many factors such as engine system, test control, cylinder head casting, heat treatment, raw materials, machining, etc. This article focuses on the abnormal cooling system of a certain diesel engine during the cold and hot shock durability test. Through clear investigation and confirmation, it is found that the cracking occurred inside the cylinder head, and the specific location of the cracking is identified. Through a series of failure analysis and testing, including cylinder head material, performance, macroscopic and optical microscopic examination of the fracture surface, and casting residual stress, the true cause of the cylinder head cracking is analyzed. The main defects are surface defects of the cylinder head casting, including micro-pores and micro-shrinkage porosity. At the same time, casting residual stress and small R-angle at the root of the water jacket are also factors that promote crack formation, and improvement suggestions are proposed.