In order to find a rapid and accurate numerical method to compute the multipactor threshold in microwave device, three enhanced Monte-Carlo (MC) methods are proposed which are single particle-multiple collision MC, multiple particle-single collision MC and multiple particle-multiple collision MC method. The three MC methods all give the random nature of the secondary electrons, including their initial energies, phases and angles. And in all of the methods, the electron trajectory is computed with Runge-Kutta method and the secondary electron yield (SEY) per collision is computed with Furman model. The effective SEY is taken as the criterion to judge whether multipactor occurs, the definition of which is a little different from those of the three MC methods. As a verification, the multipactor in a parallel plate transmission line is investigated with the presented MC methods and the traditional MC method. The numerical results of the four MC methods are compared with the results of the statistical theory. It is demonstrated that the single particle-multiple collision MC method has the smallest error and the best stability.