The occurrence and saturation of electromagnetic (EM) instability in femtosecond laser-plasma interactions are investigated using relativistic particle-in-cell (PIC) simulations. The linear and nonlinear saturations of EM instability are discussed. It is found that for plasma of non-Maxwell velocity distribution, the initial random thermal motions of electrons induce EM instability in plasma, and the resulting intense EM field causes the deposition of energy on electron beams in a very short distance and suppresses the energy transportation of super-thermal electrons formed by the ponderomotive force of the laser. This research benefits the understanding of mechanism of self-generated magnetic fields in inertial confinement fusion (ICF).