The utility model provides a concrete reinforcement protective layer pre -detection instrument for roof beam, post, wall, including a A board, the central authorities of A board connect an action bars perpendicularly, the width of A board is M -P, and wherein, M showes protective layer nominal thickness, and P is the maximum permissible minus deviation of protective layer nominal thickness, still can include a B board, the other end of the perpendicular attended operation pole of central authorities of B board, the width of B board is M+Q, and wherein, M is the same with the A board, and Q is the maximum permissible plus deviation of protective layer nominal thickness. Use protective layer design thickness as 50mm's roof beam as the example, in reinforcement back well, stretch into with the wide A board of 43mm earlier and detect between reinforcing bar outward flange to the template, the part that the A board can pass through is for qualified, if the A board can not pass through, explain then that the cover to reinforcement is little, the wide B board of reuse 60mm detects, and the part that the B board if can pass through explains that the cover to reinforcement is big. Utilize the utility model discloses whether the concrete reinforcement protective layer of detection beam, post, wall accords with the designing requirement fast, has simple structure and convenient operation's characteristics.