The wavelength resolution power of a elliptically curved crystal spectrometer is discussed.After the spectral line profile caused by itself is supposed to be neglected,two main factors effecting on the wavelength resolution power of the spectrometer are analyzed.The first one is mainly that the source center is not on the focus of the ellipsoid and the source size can not be ignored.The other one is that the X-ray is reflected from irregular crystal surface, and it is not an ideal ellipsoid.In order to improve the wavelength measuring precision of the spectrometer, two simulating models are established according to the above mentioned factors.Then the two models are analyzed and the relation between the wavelength resolution power and the exit aperture slit width is presented by digital calculation.Meanwhile, a number of experimental investigations on the wavelength-resolved characteristics of the diagnostic system are conducted under a variety of slit width conditions.The wavelength resolution power affected by the un-ideal crystal surface and the source size is quantitatively analyzed with the spectrometer successfully.The experiments show that obtained results are in agreement with the calculation data.When the exit aperture slit width(2δ) is 10 mm,the full width at half maximum(Δλ) is 3.1×10-3 nm;the 2δ is 4 mm,and the Δλ is 2.3×10-4 nm.These results prove the validity of simulated results.