In classic optical communication, intensity modulation direct detection (IM-DD) is a low-power (amplitude) system with no amplifier in the intermediate. Therefore, increasing the channel capacity becomes a challenge of signal space size and distribution at given bandwidth and maximum power (amplitude). The problem of limited power is solved by proposing a dynamic probabilistic shaping (DPS) algorithm based on the IM-DD system, which improves the system's channel capacity and bit error performance. The pulse amplitude modulation (PAM) format is used in this investigation. DPS can increase channel capacity by approximately 0.9 dB and improve bit error performance by 0.8 dB and 1 dB in PAM4 and PAM8, respectively, according to simulation data.