The invention discloses an overall composite structure hard alloy gear and a preparation method thereof. The hard alloy gear comprises a mosaic portion (1) and a stress portion (2) with a working surface, the stress portion (2) is prepared from first raw materials, the mosaic portion (1) is prepared from second raw materials, the contacting boundary portion (3) of the mosaic portion (1) and the stress portion (2) is prepared from first raw materials which are high-performance original raw materials, the second raw materials are reclaimed materials or original raw materials, the grades of the reclaimed materials are as same as those of a work portion, and the grades of the original raw materials are different from those of the work portion. The preparation method of the alloy gear includes the steps: respectively filling the first raw materials and the second raw materials into an upper layer and a lower layer of a hard alloy gear mold; sintering the raw materials by dewaxing, under variable pressure and at variable temperature to obtain the hard alloy gear. The stress portion of the hard alloy gear has high strength and abrasion resistance meeting requirements, and the mosaic portion has high toughness, so that the gear can be fixed.