This paper investigates how to regulate the molecular weight of polyphenylene ether (PPO) through reactive extrusion technology. When the dosage of the initiator dicumyl peroxide (DCP) is fixed at 1.25% (mass fraction), bisphenol A (BPA), as an efficient chain transfer agent, significantly reduces the number average molecular weight (Mn) of PPO by increasing its dosage from 0.67 wt% to 4.17% (mass fraction), with Mn decreasing systematically from 12,406 g·mol-1 in pure PPO to one-third of that (e.g., F3: 4556 g·mol-1). When the BPA dosage is fixed at 4.17% (mass fraction), the effect of DCP on Mn is non-monotonic: at 1.125 wt% (S2), Mn reaches its lowest value of 2828 g·mol-1, but excessive DCP (1.25%, S3) inhibits the chain transfer efficiency due to a sharp increase in radical concentration, causing Mn to rebound to 4556 g·mol-1. Thermogravimetric analysis (TGA) further verifies the effect of molecular weight regulation: the initial decomposition temperature (T5%) of modified PPO decreases with the reduction of Mn (pure PPO: 480℃), as the increase in low molecular weight chain ends accelerates thermal decomposition; S2 (with the lowest Mn) has the lowest T5% (393 ℃). In summary, BPA can linearly regulate Mn, while DCP has an optimal threshold of 1.125% (mass fraction). Both factors, through the mechanisms of chain transfer and termination, jointly determine the molecular weight and thermal stability of PPO.