The invention discloses a method for simultaneously preparing pyrazinamide and isonicotinic acid through uncatalyzed hydrolysis of isonicotinic nitrile in a near critical water medium. The method comprises the following steps: adding deionized water and the isonicotinic nitrile into a high-pressure reaction kettle, stirring the mixture, raising the temperature to the boiling point at normal pressure, and opening an exhaust valve for 2 to 5 minutes; closing the exhaust valve, continuously raising the temperature to be between 200 and 300 DEG C, and hydrolyzing for 10 to 15 minutes; cooling hydrolysate, adjusting the pH value of the hydrolysate to be between 3 and 4, performing crystallization and filtration on the hydrolysate and obtaining coarse isonicotinic acid and filtrate; making the coarse isonicotinic acid subjected to hot-water dissolution, activated carbon decolorization, secondary crystallization and vacuum drying and obtaining isonicotinic acid products; adjusting the pH value of the filtrate to be between 8 and 9, performing crystallization and obtaining coarse pyrazinamide; and making the coarse pyrazinamide subjected to hot-water dissolution, activated carbon decolorization, secondary crystallization and vacuum drying and obtaining pyrazinamide products. The method does not add any catalyst during reaction, solves the pollution problem of acid and alkali catalyzed hydrolysis, has simple and green process, and has high purity and yield of the products.