Deep-seated gas in seabed sediments migrates upwards from effect of external factors, which easily accumulates to form gasbags at interface of shallow coarse-fine sediments. Real-time monitoring of this process is important to predict disaster. However, there is still a lack of effective monitoring methods, so we attempt to apply multi-points pore water pressure monitoring technology when simulating forming and dissipation of gasbags in sediments through laboratory experiment. This study focuses on discussion of sensitivity of pore water pressure monitoring data, as well as typical changing characteristics and mechanisms of excess pore water pressure corresponding to crack generation, gasbag formation and gas release. It was found that the value of excess pore water pressure in sediments is negatively correlated with vertical distance between sensors and gas source, and the evolution of gasbag forming and dissipation has a good corresponding relationship with the change of excess pore water pressure. Gasbag formation process is divided into three stages: transverse crack development, longitudinal cavity expansion, and oblique crack development. Formation of gasbag begins with the transverse crack at the interface of coarse-fine sediments while excess pore water pressure attenuates rapidly and then drops, pressure remains almost unchanged when cavity expanses longitudinally, oblique crack appeared in final stage of gasbag evolution while excess pore water pressure accumulated and dissipated again. The variation curve of excess pore water pressure in gas release stage has saw-tooth fluctuation characteristics, and the value and time of pressure accumulation are also fluctuating, indicating the uncertainty and non-uniqueness of gas migration channels in sediments.