The coupling coefficient between microring laser and straight waveguide is analyzed by using 3-dimension coupled mode theory. The influence of all parameters of semiconductor microring laser on the coupling coefficient and microring laser threshold is analyed by using the self-reproduce condition in combination of analytical expression of coupling coefficient. This method is simple and requires less calculation. The results show that the coupling efficiency decreases with the increase of wavegulde width,thickness and gap and it increases with the increase radius of microring laser. The threshold of microring laser increases with the increase of waveguide width,thickness and waveguide gap,and it decreases with the increase of the microring radius.