The present invention provides a modified cellulose gel breaking agent for a drilling fluid and a preparation method thereof, and the modified cellulose gel breaking agent comprises natural cellulose, a modifying agent, an initiator and a pH regulator. The modified cellulose gel breaking agent for the drilling fluid is prepared from natural cellulose as a raw material, the modified cellulose gel breaking agent can be used for gel breaking and destabilization of a low solid polymer drilling fluid for a first opening well section of an oil-gas well of Changqing Oilfield, and solves the problem of no place for emission of the drilling fluid after implementation of no falling of the drilling fluid on the ground. The modified cellulose gel breaking agent has no biological toxicity, is biodegradable and environmentally friendly, the used raw material is low in cost, the preparation method and required equipment conditions are simple and easy to implement, the requirements of clean production on the gel breaking agent for the drilling fluid in the large scale drilling process in the Changqing Oilfield can be met, production cost can be greatly reduced, zero discharge and zero pollution of the drilling fluid can be realized, and the modified cellulose gel breaking agent has good economic benefits, environmental benefits and social benefits.