The invention discloses a method for extracting shale oil gas by an oil shale in-situ topochemical method. The method includes: a partial chemical reaction area is formed in an underground oil shale stratum via hot mixed gas; in the whole process, a series of 'chain reactions' are induced by means of controlling concentrations of injected hot mixed gas and recovered hot mixed gas, and along with gradual temperature rising and range widening of the reaction area, porosity and permeability of the oil shale stratum increase continuously, heat strengthening reaction processing is realized finally, and self catalytic cracking of the oil shale stratum from inner to outer is achieved to generate shale oil and fuel gas. After the chain reactions are over, fixed carbon in the oil shale stratum is continuously enabled to react to generate low-heating-value gas via the hot mixed gas, high-temperature waste heat in the area finished in cracking reaction is recycled, and self-supporting and maximal utilization of energy for the oil shale stratum cracking are realized. By the aid of the method, exploring method, environmental harms and commercial risks can be reduced.