In order to improve the multiplexing capacity of fiber sensing system, several sensors with overlapped spectra are always connected in series. However, crosstalk effects introduced by serial connections will lead to measurement errors, which means that crosstalk effects between sensors should be well researched. The Monte-Carlo method is introduced to analyze the system composed of several serial sensors with overlapped spectra. This method can take crosstalk effects like spectrum shadowing and arbitrary-order multiple reflections into account and calculate the reflectivity spectrum of the system. In order to verify the reliability of the Monte-Carlo method, reflectivity spectra of a system composed of two serial sensors are calculated by the analytic solution and the Monte-Carlo method, respectively. As the number of light beams set to be 5×106, the calculate error level is about 10-3. The comparison indicates that the two methods have a good agreement, approving the accuracy of the Monte-Carlo method. The reflectivity spectrum of a system composed of more than two serial sensors which have no analytic solution is also analyzed by the Monte-Carlo method.