Based on classical electromagnetic theory,the conservation law for linear momentum in an optical field is derived. The expressions of gradient and scattering forces exerted on an arbitrary object are obtained. It validates that electromagnetic model can be used to explain fundamental of laser trapping and manipulation of living biological cells and organelles with cells. Theoretical analysis indicates that the forces acting on Rayleigh particles can be divided into two types: the scattering force which comes from electromagnetic wave momentum changes due to the scattering by a dipole and the gradient force which is equal to the Lorenz force acting on the induced dipole. And the optical forces are relative to systemic parameters including the radius of the beam waist,the relative refractive index of particle and surrounding medium,the radius of particle and laser power.