An assembling method of an injection mold includes the steps of: disposing guide rods and a ejection pin plate on a punch template; inserting guide bushes into the ejection pin plate; inserting ejection pins respectively into the ejection pin plate and the punch template; fastening a ejection pin fixing plate to the ejection pin plate; fastening a die fixing plate and spacer plates to the punch template, and inserting the other ends of the guide rods into the die fixing plate; and disposing a press plate into the die fixing plate, and fastening the guide rods to the die fixing plate by tightening screws. The assembling method set the guide rods first o achieve a positioning function before ejection pin plate is assembled, and thereby capable of effectively eliminating the problems of deformation of the ejection pin plate, breaking of the ejection pin, and abrasion occurring in the return stroke.