We study nonlinear physical characteristic of a semiconductor laser due to external optical injection. By small-signal analysis, we analyze dynamically behavior of the laser. Effect of the external light injection on nonlinear frequency chirping is analyzed and static maximal locking formula is given. We indicate theoretically that the frequency chirping of the optical injection laser is nonlinear related to the external injected light and the physical state of the laser while this nonlinear effect is promoted with adding the optical injection index. And the nonlinear frequency chirping can be nonlinearly affected by the frequency detuning and the linewidth broadening factor. Obviously, the nonlinear frequency chirping is physically different from the frequency chirping of the absolute laser. The frequency detuning, the maximal locking region, the bifurcation condition and bifurcation period formula are introduced. Dynamical behavior from bifurcation to chaos is numerically simulated with the external optical frequency detuning and the injection parameter, in detail. A single period, a tri-period, a multi-period, chaos, fully developed chaos and their rotating whirl attractors, frequencies, spectrums and waveforms are analyzed numerically, respectively. The maximum Lyapunov exponent, static maximal locking region and bifurcation period are calculated numerically, respectively. Bifurcation of the laser with the frequency detuning is numerically simulated. Unstable dynamical behavior and spectrum characteristic of the laser system is comprehended gradually.