The invention discloses a fit dynamic peak searching method for detecting gas concentration by laser, which comprises that: a controller adjusts the working temperature of a wavelength specific laser to a certain proper value, linearly increases bias current for the wavelength specific laser, and simultaneously receives current signals transmitted by a photoelectric detector to obtain a photocurrent primitive curve; the controller judges whether an absorption peak is in the middle of the photocurrent primitive curve, if the absorption peak is not in the middle of the curve, the temperature adjustment is continued, otherwise, the photocurrent primitive curve is subjected to secondary differential analysis to form a secondary differential curve, and proper fit points are selected to fit a reference curve; and the controller compares the reference curve with the primitive curve to obtain a normalized absorption peak curve, and the gas concentration is obtained by calculating the maximum of the normalized absorption peak curve. Through the method, the position of the absorption peak of the gas to be tested can be dynamically found in real time, and the method for searching the absorption peak has the advantages of strong real-time and accurate peak searching. The normalized absorption peak curve obtained by secondary differential analysis and curve fit eliminates the influence of system noise and wavelength slow drift, and the concentration of the gas to be tested deduced by the normalized absorption peak curve is more accurate.