Based on a theoretical analysis of the nonlinear polarization rotation (NPR) mechanism, we propose a simplified method to locate the soliton mode-locking region in an erbium-doped fiber laser (EDFL), enabling efficient automatic startup without extensive random exploration of polarization states. A novel correlation between the NPR transmission curve and the mode-locking threshold is established, showing that under specific NPR transmission curve the threshold can be reduced to 35 mW with soliton-only output, and stable soliton operation is sustained by increasing the pump power from 35 mW to 134 mW at a fixed polarization state. By combining the NPR mechanism with an Actor-Critic reinforcement learning algorithm, the approach enables automatic identification and stable maintenance of soliton mode-locked states. The laser generates stable pulses at a repetition rate ～ 11.9 MHz with an average intracavity power ～ 43.74 mW, corresponding to a pulse energy ～ 3.68 nJ and the pulse peak power of 3.2 kW. The proposed approach offers stability, repeatability, and adaptive capability, providing a practical route toward high-energy pulse generation and reliable access to soliton dynamics.