The ultrasonic-assisted modification of novel silkworm-excrement-based porous carbon (PCSE) with various Lewis acid metal ions (LAMIs) was prepared and applied for the sustained release of the pesticide thiamethoxam (TM). The effects of the LAMIs on the TM adsorption and release mechanism were analyzed and investigated using hard and soft acid–base theory (HSAB). The results showed that the prepared PCSE showed a micro/mesoporous structure and its adsorption capacity for TM reached 616.8 mg g⁻¹ at 298 K. The LAMIs were coordinated with acid oxygen groups on the PCSE and were loaded as the corresponding metal oxides with a lower valence under an ultrasonic field. The low valence of the LAMIs significantly decreased the interaction between TM (hard base, χ = 4.08) and the LAMI-modified PCSEs due to the decrease in the local acidity of the PCSE. Thus, the release amount and release rate of TM from the LAMI-modified PCSEs was successfully enhanced in the order of Fe³⁺/Fe²⁺ < Cu²⁺/Cu⁺ < Ag⁺/Ag. As a result, the obtained PCSEs modified by LAMIs showed a sustained release rate of 17.7–19.7 mg (g⁻¹ day⁻¹) for the first 10 days and a slow release rate of 2.2–2.3 mg (g⁻¹ day⁻¹) within the next 30 days. According to actual TM usage conditions for some crops (1.0–2.0 mg m⁻²), only 0.50 g of TM-loaded PCSE can efficiently control the main pest of crops per square metre and can last for over 40 days.