A new model is built to predict the width of ablated channel and calculated the ablation threshold of materials which are processed by femtosecond laser. In order to verify the accuracy of the model, a 50 MHz femtosecond laser with different scanning speeds is used to process the target. The micromorphology of the processed region consists of an ablated channel and a resolidification line. The widths of the ablated channel and the resolidification line are measured. The predicted results of the ablation channels are compared with the experimental results. The calculated ablation thresholds are compared with the results which are calculated by the previous method, too. Both errors are negligible. The model can be used to calculate the widths of the ablated channel and the resolidification line at other scanning speeds.