Abstract: In laser deep penetration welding, the laser incident angle directly affects the shape of the keyhole, and also has a certain impact on the flow behavior of the molten pool. In this work, bead-on-plate penetration welding experiment combined with "half sandwich" experiment were conducted to study the effect of laser incident angles on weld mechanical properties and melt pool flow. The results show that in terms of the mechanical properties of the weld, the laser incident angle has less impact on the upper and middle parts of the weld, but has a greater impact on the bottom part. There are more pore defects in the tensile samples from the bottom of the weld. Moreover, compared with negative laser incident angle, the tensile strength and elongation of the weld are lower for positive laser incident angle. For positive laser incident angle, the keyhole morphology is of small top and big bottom, makes it easy to generate eddy currents at the bottom and form bubbles. In addition, the keyhole collapse frequency is greater than that at the negative laser incident angle, resulting in poor stability of the molten pool which leads to more pore defects