To quantitatively describe the human reliability of manual station on engine assembly line within different working days and operation times, a model was proposed considering the shortest cognitive process, cognitive fatigue, and the shortest operation process. Based on the learningforgetting and fatiguerecovery theory, the cognitive time was developed with the shortest cognitive process and cognitive fatigue, while the operation time was developed with the shortest operation process. According to the cognitive time and operation time, fatigue failure rate was calculated and cognitive failure rate was introduced with the HCR(human cognitive reliability) method. After obtaining the failure coefficient according to their time variance, a human time variant reliability model was built. A case study verified the feasibility of the model. The result can be used in human reliability evaluation and product prediction.