The nonlinear dynamics of a weak-resonant-cavity Fabry-Perot laser diode subject to optical injection was experimentally investigated. By recording the time series, power spectra and optical spectra of three longitudinal modes (“mode －3”, “mode 0”, and “mode 13” ) in the weak-resonant-cavity Fabry-Perot laser diode, the nonlinear dynamical states have been determined. The results show that, after introducing optical injection, through varying injection strength and frequency detuning, the three longitudinal modes can exhibit four-wave mixing, period one, quasi-period, chaos, and stable injection-locking, and the dynamical evolution route is from quasi-period to chaos. Moreover, in order to control the dynamical state accurately, we draw the dynamical state mappings of the three longitudinal modes in the parameter space of injection strength from 0 μW to 450 μW and frequency detuning from －16 GHz to 16 GHz. The results demonstrate that the chaotic region can be observed in the positive and negative frequency detuning for “mode 0” and “mode 13”, but cannot be observed in the negative frequency detuning region for “mode －3”. And there exist two separate stable injection-locking regions for “mode 0”, but there is only one stable injection-locking region for “mode －3” and “mode 13”.