A novel hybrid surface plasmon waveguide structure with rounded metal ridge and low refractive index air gaps is designed. The changes of electric field distribution, normalized mode area, transmission distance, quality factor and Purcell factor with radius of metal ridge curvature are studied at operating wavelength of 489 nm. The results show that the waveguide can achieve a larger radiation enhancement factor while ultra-deep subwavelength optical field can be limited by adjustment of structure parameters. The optimal geometric parameters are as follows: nanowire radius and curvature radius are 95 nm and 20 nm, respectively. Under the optimal parameters, effective mode area is 0.0037λ2, quality factor is 268, Purcell factor is 65 and gain threshold is 0.2768 μm-1. The value of the laser enhancement is calculated as 69800. Resonant cavity of the laser has superior local ability and laser enhancement ability, and can realize low threshold excitation of ultra-deep subwavelength.