There is a "pit" phenomenon in the target surface when long pulse laser ablates the foil target and speeds up the material of target. Laboratorty diagnosis and theoretical simulation of the pit can help us understand the relevant physical processes and check relevant parameters of the procedures. We use 13.9 nm Ni-like Ag X-ray laser as a probe to diagnose the plasma and the pit phenomenon, which is produced by 2ω nanosecond laser irradiation of C8H8 foil target. XRL2D code is used to simulate the experiments. Results show that, when the limiting factor of electron thermal transmittance is 0.03, the simulated pit width and depth (the distance of thin foil accelerated)agree with the results of experiment, but the foil thickening phenomenon in the area far away from the focal spot region cannot be explained the simulation.