According to the large junction capacitance of the rib waveguide pn junction of the single-drive push-pull silicon modulator, which limits the further improvement of the modulators bandwidth, a silicon modulator with slot rib waveguide pn junction structure is proposed. By etching the rib waveguide pn junction with a certain size of slot, the capacitance of the pn junction is reduced and the modulation bandwidth is increased. The two pn junctions are simulated and compared, and the results show that with the same electrode structure and doping conditions, the electro-optical bandwidth of the slot rib waveguide modulator is increased by about 8.3GHz compared with the rib waveguide modulator. Based on the optimized single-drive push-pull slot rib waveguide modulator, the 90Gbit/s OOK signal modulation is simulated, and 14.69dB extinction ratio of eye-diagram is obtained.