The invention relates to the technical field of clay swelling inhibition, and especially relates to a preparation method of a heavy oil thermal recovery well clay swelling inhibitor. The method comprises the following steps: 1, adding 4 parts by mass of polyethylene polyamine and 11 parts by mass of water into a closed reaction kettle, uniformly stirring the polyethylene polyamine and the water for 30 min, and heating the obtained solution to 50 DEG C; 2, adding 3 parts by mass of epichlorohydrin into the closed reaction kettle, uniformly stirring the heated solution and the epichlorohydrin for 1 h, heating the obtained solution to 70 DEG C, carrying out a reaction for 4 h, and cooling the obtained reaction product to 20 DEG C; and 3, adding 2 parts by mass of potassium chloride into the closed reaction kettle, and uniformly stirring the cooled reaction product and the potassium chloride for 1 h to prepare the clay swelling inhibitor. The clay swelling inhibitor still can inhibit clay swelling in steam injection working high temperature environment, meets heavy oil thermal recovery well steam soaking temperature environment requirements, and has the advantages of good high temperature resistance, prevention of clay migration, simple preparation method, convenience in onsite use, and guaranteeing of the thermal recovery well steam injection effect and the stable output.