The invention discloses a nickel selenide and ternary nickel selenide iron composite electrocatalyst. The nickel selenide and ternary nickel selenide iron composite electrocatalyst comprises foam nickel iron alloy, and a layered electrocatalyst layer formed on the surface of the foam nickel iron alloy through in-situ growth; the layered electrocatalyst comprises NiSe2 and NiFe2Se4, and is of a three dimensional nanometer wrinkle structure. The invention also provides a preparation method of the composite electrocatalyst. The preparation method comprises following steps: nickel iron alloy is subjected to ultrasonic cleaning; selenium powder is introduced at the bottom of a quartz tube, the foam nickel iron ally is fixedly arranged at the middle part of the quartz tube, and vacuum sealing ofthe quartz tube is carried out; the quartz tube is introduced into a tubular furnace for calcining, so that in-situ growth of the layered electrocatalyst of the three dimensional nanometer wrinkle structure on the surface of the foam nickel iron alloy is realized. The invention also provides applications of the composite electrocatalyst in electrocatalytic water decomposition to prepare oxygen under alkaline conditions. The layered electrocatalyst is of the three dimensional nanometer wrinkle structure, so that the electrochemical area of the composite electrocatalyst is enlarged, and high activity and stability on electrocatalytic oxygen evolution reaction are achieved.