Abstract: This paper studies the effect of the viscosity of LiNi_0.5Co_0.3Mn_0.2O₂ cathode material on lithium ion vehicle power battery. The effect of different NMP content on the viscosity of the slurry leads to its effect on the uniformity of battery and electrochemical performance. The optimum formulation of NMP content was utilized to study the effect of different stirring time on the viscosity of the cathode. Viscometer, internal resistance tester, accelerated adiabatic calorimeter, battery test system are applied to characterize slurry viscosity, battery internal resistance, battery thermal stability and electrochemical performance. The experimental results show that NMP content of 880 g and stir for 6 hours, the average viscosity of 6.348 pa﹒s, with slight internal resistance difference and capacity difference. When cycled at 25 ℃with 0.5 C in the voltage range of 2.75-4.2 V, the first discharge capacity of 163.5 mAh/g with capacity retention rate of 98.6 % after 100 cycles. under the adiabatic conditions, the highest temperature of 47.3 ℃when battery charging and discharging process. Therefore, it shows good battery consistency, electrochemical performance and thermal stability.