Abstract: The vertical inline pump is a centrifugal pump with the elbowed inlet pipe, which has excellent characteristics of simple structure, small volume, easy for installation, and so on. Therefore, it is widely applied in where the constraint is installation space. However, the curved inlet structure also has a negative impact on the inflow conditions before the impeller, which will result in the complex flow phenomenon and decrease of the pump performance. In order to study the features of flow distortion and its impact on the performance and stability of the inline pump, the three-dimensional unsteady Reynolds average Naiver-Stokes equations with the shear stress transfer model were solved by commercial CFD code for the selected pump model in this study. Meanwhile, the Bezier curves were adopted to fit the profiles of the curved inlet pipe and the impeller. Thirty-nine coordinates of the control points of those Bezier curves and the number of the impeller blades were selected as the design variables for the parametric design of the inlet pipe and the impeller. Based on the Latin Hypercube Sampling method, 300 groups of cases were generated in the decision space, and the influence of these design variables on the inflow features and the performance of the inline pump was studied based on Pearson correlation analysis. In order to ensure the reliability of the numerical simulation, a validation experiment on the original pump was carried out. The comparison between the computational results and experimental results showed that the calculation has good accuracy on flow prediction, which could meet the requirements of further study. During the numerical investigation on the original case, a large flow separation area with a pair of secondary flow vortices was found in the inlet passage, which extended along the outer side of the inlet pipe, blocked the flow passage seriously, and deteriorated the outflow conditions of the inlet pipe. Under the nominal and part-load conditions, the main flow features in the inlet pipe were reverse vortex pair and backflow vortex, respectively. The correlation analysis results showed that the performance of the inline pump is significantly related to the blade shape and the number of blades. Under the nominal condition and overload conditions, the increase of the blade angle near the leading edge is beneficial to improve the efficiency and head while the increase of the blade outlet angle has only a positive effect on the lift of the head. Under the part-load conditions, the effect on the performance of the inlet blade angle is significantly reduced and the correlation between the outlet blade angle and the characteristics of the inline pump is prominent. It also reported that the cases with the inlet blade angle of 40°-50°, the outlet blade angle of 20°-40° and longer inlet pipe usually have better performance and stability, which have an average efficiency increase of 5% compared with the original case. The research can provide some reference for the design optimization of inline pumps.