NOVELTY - Purifying and recycling secondary air in thermal flood comprises (i) subjecting secondary gas to gas-liquid separation, conveying raw material gas into a compressor, conveying the raw material gas into a preprocessing absorption tower from the bottom of preprocessing absorbing column, processing selective adsorption using a preprocessing adsorbent, and removing substantial part of heavy hydrocarbon impurities in the raw material gas; (ii) entering oil removed raw material gas into the pressure swing adsorption (PSA) decarburization, and decarburizing; and (iii) performing PSA denitrification.    USE - The method is useful for purifying and recycling secondary air in thermal flood (claimed).    ADVANTAGE - The method: can concentrate and recycle methane in the secondary gas generated in thermal flood through optimal process flow with PSA treatment method; produces the product gas with high concentration; obtains methane with purity of 90-99.999%; has short process; and solves the problem that carbon dioxide and nitrogen are difficult to separate.    DETAILED DESCRIPTION - Purifying and recycling secondary air in thermal flood comprises (i) preprocessing step: subjecting secondary gas to gas-liquid separation, conveying raw material gas into a compressor under pressure of 0.6 MPa, conveying the raw material gas into a preprocessing absorption tower from the bottom of preprocessing absorbing column of preprocessing system, processing selective adsorption using a preprocessing adsorbent, removing substantial part of heavy hydrocarbon impurities in the raw material gas, and then conveying purified gas into pressure swing adsorption (PSA) decarburization system; (ii) entering oil removed raw material gas into the PSA decarburization system, decarburizing the adsorbent of the selectively adsorption on the absorption tower in the raw material carbon dioxide gas, emptying the mixed gas through a vacuum pump to remove carbon dioxide, and conveying top gas into a PSA denitrification system; and (iii) conveying the decarburized gas into the PSA denitrification system through the bottom of the absorption tower, using a denitrification adsorbent of methane gas, removing nitrogen in the mixed gas, emptying the methane-rich gas by the vacuum system, and conveying into a buffer tank, compressing through methane compressor to 0.35 MPa.G, and conveying to the areas, where the secondary gas mainly comprises nitrogen, carbon dioxide and methane and further comprising hydrocarbons (C5+) and heavy impurities.