Provided is a three-phase dry-type transformer. The three-phase dry-type transformer comprises three single-phase dry-type transformers connected to each other. Each single-phase transformer comprises an incoming line side high-voltage coil and an outgoing line side high-voltage coil, wherein the incoming line side high-voltage coil is provided with an incoming line end and an outgoing line end, and the outgoing line side high-voltage coil is provided with an incoming line end and an outgoing line end. The outgoing line end of the incoming line side high-voltage coil is connected to the incoming line end of the outgoing line side high-voltage coil, such that the incoming line side high-voltage coil and the outgoing line side high-voltage coil are connected in series. Further provided is an assembly method.