Measurement of the carrier transport in the GaAs photoconductive semiconductor switch (PCSS) is important for the study of the GaAs PCSS current filament and the design of improved GaAs PCSS devices. In this study, an optical testing method was proposed for the measurement of the time-dependent carrier density distribution in the GaAs PCSS. The measured electron density curves show that the carrier density distribution between the two electrodes is nonuniform for Δt = 0, and as Δt increase from 0.6 ns to 6 ns, a large number of carriers concentrate near the anode of the GaAs PCSS. These phenomena are analyzed in detail and the measured curves are found to be in good agreement with the simulation results.