Biomass-based interconnected porous carbon is not only inexpensive and eco-friendly, but also has the characteristics of a large specific surface area and multiple heteroatom co-doping, making it prominent in the energy storage device field. This work reports a simple method to fabricate N, P, and S co-doped interconnected porous carbon from bamboo pulp fiber by one-step pyrolysis method with NaOH/thiourea aqueous and melamine phosphate. In this reaction system, NaOH/thiourea is an activator and a solvent for bamboo pulp fiber, thiourea is the dopant of nitrogen and sulfur, and melamine phosphate is the dopant of nitrogen and phosphorus. The synergistic effect of NaOH/thiourea and melamine phosphate under freezing conditions contributed to increasing the specific surface area, mesoporosity, degree of graphitization, and heteroatom doping of porous carbon, thereby improving its electrochemical behavior. The synthesized porous carbon exhibits interconnected loose structure, huge specific surface area, extremely high mesoporosity (98.26%) and N (3.78 at%), P (0.32 at%), and S (3.36 at%) co-doping characteristics. As a supercapacitor electrode, it presents a specific capacitance (316 F⋅g⁻¹) at a current density of 1 A⋅g⁻¹. After assembly into a symmetrical supercapacitor, it has a high energy density (16.3 Wh⋅kg⁻¹) at a power density of 489.0 W⋅kg⁻¹, and an excellent cycling stability (91.2% capacitance retention after 5 000 cycles in 6 M KOH). In addition, this work aims to find a novel sustainable approach of using renewable biomass to produce high performance electrode materials to meet the need of different energy storage applications.