Alumina ceramics are widely used in the field of electronic packaging because of their excellent dielectric properties and stable physical and chemical properties. However， the large crack tendency of its weld reduces the joint performance and greatly limits the application of ceramic joints. In this paper， a femtosecond laser is used to weld alumina ceramics. The effects of laser beam scanning trajectory， laser power， and scanning speed on weld crack characteristics and joint properties are studied. A classification method of weld grade based on average crack width is proposed. The results show that compared with ellipse， helix， vertical 8 characters， and horizontal 8 characters， sinusoidal scanning laser welding alumina ceramics can obtain smaller average crack width and higher shear force. Increasing the laser power and decreasing the scanning speed is beneficial to reduce the average crack width， increase the weld penetration， and then increase the shear force of the joint. The maximum shear force reaches 1980 N， which is about 61% of the base material shear force. A prediction model of joint shear force based on the average crack width and penetration depth of the weld is established， and the model’s accuracy is proved by verification experiments. The minimum deviation between the predicted value and the measured value is only 0.45%. The research results provide technical guidance and theoretical support for high-performance welding of ceramics.