Abstract: The phase screen theory has been used quite extensively in ionospheric scintillation work as well as interplanetary and interstellar scintillations. With the electron density variation of background ionosphere along the propagation path of signal taken into account, on the basis of the electron density profile and the total electron content (TEC) of the background ionosphere available from international reference ionosphere (IRI), the scintillation characteristics of transionospheric Global Position System (GPS) signal are simulated numerically by using multiple phase-screen model with different parameters for each layer. A comparison of the multiple phase-screen results with the thin phase-screen results is made. The emphasis is on the equivalent ionospheric thickness for the thin phase-screen model.