The invention relates to a catalyst and application thereof. The catalyst comprises a chromium compound a, an N-containing compound b, an electron donor c and a promoter d. The mole ratio of the four components is 1: (2-10): (8-100): (40-2000). The catalyst is used for catalyzing ethylene oligomerization to composite decene. In an ethylene oligomerization high-pressure reactor, the components of the catalyst are subjected to catalyst in-situ preparation in an inert solvent according to certain effective quantities; ethylene is injected continuously, and the catalyst is enabled to fully contact with the ethylene to achieve an oligomerization reaction. The decene selectivity of the catalyst is higher than 50%.