A coaxial assembly includes a first member, a second member, and at least two position members. The first member has an assembly hole with a sidewall. The second member having a lateral wall. The at least two position members are evenly arranged along a circumference of the lateral wall of the second member. The second member and assembly hole are configured such that when the second member is within the assembly hole, the at least two position members tightly abut the sidewall of the assembly hole, thereby causing the second member to be coaxial with the first member. An assembly method is also provided.