A irregular D-shape multimode fiber is used to produce low degree coherent laser.The contents include the theoretical analysis of the conduction model of D-shaped irregular multimode fiber,the numerical simulation of the light trajectory and the light intensity distribution of the D-shaped irregular multimode fiber.The experiment generates a partially coherent laser with a D-shaped fiber as a transformation component which has a length of 500 mm and core diameter of 200 μm.The coherence degree of the output is significantly lower than that of the cylindrical step-index fiber.