A method and a system of determining the deviation of dynamic position are provided. The determining method includes the follow steps. First, a server receives a plurality of first position data from a first mobile device. Next, the server receives a plurality of second position data from a second mobile device. Then, the server transmits the first position data to the second mobile device. Thereon, the server transmits the second position data to the first mobile device. Later, the server determines whether the first mobile device and the second mobile device deviate from each other according to the first position data and the second position data.