A method for assisting in focal length detection is applicable to a digital camera having the flash. The method includes the following steps. First, the flash of the digital camera is actuated and a first image is captured. Afterwards, a characteristic exposure value of the first image is calculated. A focus range comparison table is looked up according to the characteristic exposure value to obtain an initial focus position. Then, a focus procedure is performed according to the initial focus position to obtain a target focal length.