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Heteroatom doping is considered an effective method to substantially improve the electrochemical performance of Ti3C2Tx MXene for supercapacitors. Herein, a facile and controllable strategy, which combines heat treatment with phosphorous (P) doping by using sodium phosphinate (NaH2PO2) as a phosphorus source, is used to modify Ti3C2Tx. The intercalated ions from NaH2PO2 act as “pillars” to expand the interlayer space of MXene, which is conducive to electrolyte ion diffusion. On the other hand, P doping tailors the surface electronic state of MXene, optimizing electronic conductivity and reducing the free energy of H+ diffusion on the MXene surface. Meanwhile, P sites with lower electronegativity owning good electron donor characteristics are easy to share electrons with H+, which is beneficial to charge storage. Moreover, the adopted heat treatment replaces –F terminations with O-containing groups, which enhances the hydrophilicity and provides sufficient active sites. The change in surface functional groups increases the content of high valence-stated Ti with a high electrochemical activity that can accommodate more electrons during discharge. Synergistic modification of interlayer structure and chemical state improves the possibility of Ti3C2Tx for accommodating more H+ ions. Consequently, the modified electrode delivers a specific capacitance of 510 F g−1 at 2 mV s−1, and a capacitance retention of 90.2% at 20 A g−1 after 10,000 cycles. The work provides a coordinated strategy for the rational design of high-capacitance Ti3C2Tx MXene electrodes.