Whether fish can overcome water velocity barriers affects their passage at manmade fishways. By integrating hydraulics, fish behavior, geographic characteristics and environmental factors, we simulated how fish pass through different water velocity barriers. We studied the software FishXing and focused on the software's main module and key factors in fish swimming simulation, using the silver carp which is native to China, as test object for simulation and analysis. These simulation will help to improve the design of fish passage. FishXing is a unique software tool for the assessment and design of culverts for fish passage in the United States. It models the complexities of culvert hydraulics and fish performance for a variety of species and configurations. The software models fish swimming performance against culvert hydraulics across a range of expected stream discharges, slopes with different substrate and perch heights. Water surface profiles can be calculated for a variety of culvert shapes using gradually varied flow equations. The program then designates the swimming mode or swimming velocity or leap behavior according to current velocities. The output includes tables and graphs summarizing the water velocities, water depths, and outlet conditions, and the list of factors that limits fish passage for each culvert at a specific flow condition. It can also generate video-based animations. This study tested if FishXing can be used to estimate the swimming performance of silver carp. In our experimental tests, the water velocity in a open channel flume was increased from 0.4 m/s to 1.2m/s at 0.2m/s intervals, which extended beyond the range of critical swimming speeds recorded in juvenile silver carp. A camera was set above the water tank to record fish swimming throughout the experiments. Silver carp were tested one at a time and we recorded whether they could swim across a 1m distance at a given the water flow within 5 min. The FishXing software was then used to evaluate the performance of silver carp at the same values of water flow velocity. The experimental tests indicated that the success of fish at crossing velocity barriers decreased with velocity while the passage rate of fish in the FishXing software remained 100% until the flow far exceeded their critical swimming speed. The results indicate that the FishXing software does not accurately simulate how silver carps cross water velocity barriers. The analysis shows that FishXing has advantages in combing geographic characteristics, flume information and discharges. However, the software has shortcomings in that it lacks a classification of fish by swimming mode such as sustained or prolonged or burst swimming mode. There is a differentiation among different life stages and an analysis on specific behavior according to hydraulics. Further, FishXing does not accommodate non-salmonid species. We conclude that the simulation of fish crossing water velocity barriers needs to explore specific fish behavior in relation to hydraulic factors and environmental factors for different species, together with the swimming behavior of different development stages of the target fish. In the future, we should pay special attention to the swimming mode and behavioral characteristics of different species, particularly coupling swimming behavior and hydraulic characteristics.