We propose and experimentally demonstrate an on-chip all-optical silicon photonic crystal nanobeam cavity (PCNBC) modulator. With the advantages of the strong two-photon absorption (TPA)-induced thermo-optic (TO) effect, ultrahigh thermal-efficient tuning with π phase shift temperature difference ΔTπ of 0.77°C and power Pπ of 0.26 mW is implemented. Moreover, the all-optical modulation is carried out by a pulsed pump light with an average switching power of 0.11 mW. The response times for the rising and falling edges are 7.6 µs and 7.4 µs, respectively. Such a thermal-efficient modulator is poised to be the enabling device for large-scale integration optical signal control systems.