A hydraulic pile hammer with an insertion type variable diameter pile casing core comprises a hammer body, a connection flange, an anvil and a hammer casing, wherein the anvil and the hammer casing are connected into a whole through the connection flange and the hammer body; the anvil which is in a free state is arranged between the hammer body and the hammer casing; the hammer casing is fixedly connected with a pile casing core through a fastening part to enable a pile hammer to be smoothly placed on a steel pile and to be contacted with the surface of the anvil; the pile casing core is in a pipe-shaped structure; one end of the pile casing core is provided with a guide ring; and the bottom of the hammer body is provided with a plurality of seats which are used for transportation. The hydraulic pile hammer with the insertion type variable diameter pile casing core has the advantages of achieving safe and rapid detachment and replacement in an operation field according to common variable diameters; achieving rapid utilization in operation and saving large amount of steel materials, capital and engineering costs; and enabling the variable diameter circumferential degrees to be controllable, reducing operating risks, achieving protection for original pipe casing basic materials and improving the field working efficiency.