The invention brings forward an ocean platform, its crane apparatus and an arrangement method of the crane apparatus on the ocean platform. The ocean platform comprises a platform main-body and spud legs arranged on the platform main-body. The spud legs can extend out of the platform main-body and can lift relative to the platform main-body. The crane apparatus comprises a crane, a derrick post cylinder and an anti-rotating part. The crane has a derrick post. The derrick post cylinder can be fixed to the upper end of a spud leg and comprises a cylinder wall with the top end opened and a cylinder bottom formed at the bottom of the cylinder wall. The derrick post is contained inside the derrick post cylinder. The anti-rotating part is used for preventing the derrick post from rotating relative to the derrick post cylinder. The crane apparatus has a wide range of hoisting and doesn't occupy the area of the platform main-body.