We analyze the causes of longitudinal cracks from macroscopic and microscopic aspects in selective laser melting. The result shows that the mainly reason is the low energy of laser scanning. Then, testing the performance of the scanning optical system, the problem is focused to the optical system delays. To ensure that the cracks disappeared on the printed parts, adjusted the parameters of the optical system delay. Finally, according to the results of the samples (the density of the samples is 99.7% and the tensile strength is 626 MPa), to verified the laser too low scanning energy at this position is one of the factors in the longitudinal crack of the selective laser melting.