Gas-liquid displacement is still a complex and challenging topic in the field of nonequilibrium physics and many industrial applications. In this study, pore-scale displacement of gas-liquid flow in a porous medium was investigated. A digital image analysis method was used to process the images captured by a high-speed camera. The invasion pattern demonstrated that an increase in viscosity of the displaced liquid tends to decrease the finger width and the number of fingers. The effect of liquid viscosity on the invasion velocity was also investigated. The invasion velocity changes monotonously with the viscosity, showing an opposite trend at high and low gas flow rates, which is attributed to the viscous resistance of the liquid phase and the mass balance of the gas phase. Then the invasion area was measured and the N2 displacement ratio was used to estimate the injection efficiency. The displacement ratio decreases with the increase of liquid viscosity and gas flow rate. Finally, the invasion dynamics was studied. It revealed that the gas tip moves in a stepwise way, i.e., the tip goes forward, and then it stays there for a while and then goes forward again. The gas tip splits and expands in side directions and also moves backward when the gas pressure is not high enough to overcome the forward capillary pressure, which can be found for all the displaced liquids in this study.