The invention provides a device and method for quantitatively simulating the seam width dynamic change during temporary blocking in a seam. The device comprises a first chamber, a second chamber, a first piston, a second piston, a first 3D printing crack surface, a second 3D printing crack surface, a monitoring processing system, a liquid injection system and a liquid collection system. Accordingto the method, fracturing liquid and/or a temporary blocking agent is injected into a crack channel through the liquid injection system, the pressure in a mixed phase chamber is monitored in real time, and the seam width dynamic change in the process of temporary blocking in the seam is monitored in real time through the monitoring processing system. By adopting the device and the method, the typeand proportion of mixed phases in the mixed phase chamber can be designed according to reservoir rock mechanical properties and in-situ stress parameters, fine simulation of reservoir rock mechanicsand stress characteristics is achieved, then temporary blocking in the seam under the reservoir condition reappears, and the seam width dynamic change in the process of temporary blocking in the seamis quantitatively simulated and monitored in real time.