Serpentine nozzles are supplied in stealth bombers and unmanned aerial vehicles (UAVs) to evidently suppress the infrared radiation signatures (IRSs) emitted by engine exhausts. It is commonly known that excessive geometric parameters are included in the double serpentine nozzle design and, as a result, the aim of this paper is to study the influences of the design parameters on the performance of double serpentine nozzle. To this end, the design method of the serpentine nozzle was concisely introduced, and the qualifications to completely shield turbine were given. Simulations using six different turbulence models were conducted and compared to the experimental data in order to determine the suitable turbulence model for serpentine duct simulations. Then, the effects of geometric design parameters at the first serpentine paragraph exit (the dimensionless width of W1/D, area of A1/Ain, and offset distance of ΔY1/L1) on the flowfield, and the performance of double serpentine nozzle was investigated numerically. The validation study shows that the simulations with shear-stress transport (SST) κ–ω turbulence model adopted can accurately predict the flux rate, the axial thrust, and the static pressure of the experimental nozzle, and therefore, SST κ–ω turbulence model is the most suitable turbulence model in the selected six turbulence models to be used for the simulation of the double serpentine nozzles. The numerical results show that friction loss increases with the increment of W1/D due to the increased wetted perimeter, but small value of W1/D would lead to large secondary flow loss; even the shock loss appears because of the steep curvature of the second turning. Small area of the first serpentine duct A1/Ain induces high flow velocity in the first duct, which corresponds to large friction loss. Steep offset distance of the first serpentine duct ΔY1/L1 induces high local losses. As the geometric design parameters of the double serpentine nozzle interact mutually with the qualifications to completely shield the turbine, the range of design parameters should be synthetically chosen during the design progress. Thus, the width of the first serpentine duct W1/D is recommended to be from 1.0 to 1.3. The area of the first serpentine duct A1/Ain should be as large as possible, and the offset distance of the first serpentine ΔY1/L1 should be small in the permission range of the design parameters.