Abstract: Neutron velocity selector is widely used on neutron scattering instruments as key component. Developing calibration technique, designing calibration instrument and doing calibration experiment are prerequisite for its applications. Based on small angle neutron scattering instrument at China Advanced Research Reactor, the calibration instrument structure was designed and instrument parameters were determined. Count loss’s influence on measurement of wavelength resolution rate (WRR) was studied. Measurement error of WRR is decided by product of dead time and Gauss peak count rate (GPCR) and increases with GPCR for the same dead time. Calibration experiments were performed by time-of-flight method. The calculated wavelength agrees well with the experimental one by Gaussian fitting. The experimental WRRs are different from their theoretical values and increase with wavelength. The difference and the increase were caused by beam divergence. The experimental results were analyzed by theory of count loss’s influence on measurement of WRR and upper limits of neutron flux density at sample position were calculated. Neutron flux densities at sample position under different wavelengths were acquired and feasibility of two-dimensional changeable slit on flux density adjustment was checked by VITESS simulation.