The invention discloses a time synchronization method. The time synchronization method comprises the following steps: selecting a first-stage synchronous node according to the distribution condition of synchronous nodes in a time synchronization network, and setting a reference clock in the first-stage synchronous node; dividing stages of the synchronous nodes according to the topological structure of a transmission network element in the time synchronization network, and determining a cascade relationship, wherein the synchronous nodes are cascaded to at least one synchronous node in the previous stage; and performing time synchronization according to the grades, which the synchronous nodes belong to, and the cascade relationship. By means of the method disclosed by the invention, the problems that the number of transmission ports and the main clock processing capability are limited while large-scale networking is carried out can be solved; and the networking flexibility is increased; and time synchronization can be effectively carried out.