We have put forward a theory that the probation period mechanism have an information screening function when a company is inviting applications for job, which would be a supplement for the spence model. One of the merits of the model is that it could screen the informations of employee's capacity when there are fewer differences between the capacities of employees. This paper also has proved that option mechanism has a information screening effect and the compose of option and probation period mechanism could decrease the screen cost of a company. We have derived the mathematical Principles of probation period mechanism and the compose of option and probation period mechanism design in the paper.