A waterproof casing for an electronic device has a body, a cover module and multiple fixers. The body has two connecting assemblies respectively mounted on two side surfaces of the body and each connecting assembly has two wings and a trough defined between the wings and the corresponding side surface of the body. The cover module has a first cover and a second cover respectively mounted on two ends of the body. The fixers are respectively inserted into a corresponding one of the first cover or the second cove and are steadily mounted with the body. The types of the cover module can be changed to fit with an electronic device installed in the body. Therefore, the combination between the body and the cover module is variable, and the waterproof casing has good applicability.