Lidar geometrical form factor of coaxial and biaxial transmitter-receiver systems are described. Theoretical method and experimental ones for calculating geometrical form factor are presented. Some examples of geometrical form factor correction are introduced and the misalignment effects on aerosol extinction are analyzed based on many experiments of AML-1 lidar. It is concluded that the geometrical form factor is very sensitive to misalignment in the optical system, which would arise much more errors for the aerosol extinction. But the geometrical form factor will be stability when the lidar system is reliable and optical system is correct.