In order to study the characteristics of slug flow in vertical upward pipes under different viscosity conditions, experimental work and numerical simulation were carried out. The fluctuating signals of cross section pressure, liquid holdup, pressure difference and pressure drop under different viscosity were obtained and these?data?had been?processed. The results show that the length of the gas plug is reduced to 7.5-9 times of pipe diameter and the period of slug flow will be shortened as the viscosity increases, the fluctuation of pressure and differential pressure in the pipe section are changed quickly, and the average liquid holdup will ?also gradually be increased.The Mukherjee-Brill model can not accurately predict the liquid holdup of high viscosity oil-gas flow,and the VOF model has a high accuracy of liquid holdup calculation. The changes of liquid viscosity has a certain influence on the pressure drop of slug flow. The phenomenon of negative frictional pressure drop appears in both the experiment and the simulation , and the total pressure drop increases with the increase of viscosity. The computational accuracy of the Beggs-Brill model will decrease as the viscosity increases. The Hagedorn-Brown model and the VOF model predict pressure drop better, with errors below 15%, which is seen to be better than the other models.