The supersonic split line (SSSL) nozzle technique is one kind of thrust-vectoring techniques with promising application prospects. Due to the characteristics of its structural design, SSSL nozzle has obvious advantages in reducing the initial mass, compact structure and amplifying the deflection vector angle. In addition, due to its structural characteristics, the flow structure and performance characteristics in SSSL nozzle are significantly different from those in traditional nozzle. In order to study its flow characteristics and analyze the influence of different factors on the internal flow field, the internal flow field of supersonic split line nozzle with different swing angles under different design parameters was calculated and studied by using the method of numerical simulation. Results indicate that the deflection amplification effect of supersonic split line nozzle is true within the reasonable design parameters. The axial thrust coefficient and radial thrust coefficient decrease with the increase of swing angle. The larger the expansion half angle of the nozzle expansion section, the better the deflection amplification effect. The larger the expansion ratio at the split line, the better the deflection amplification effect.