In order to carry out the research of constant wavelength ultra small angle neutron scattering spectrometer （USANS）

the design principle is studied in this paper base on relevant information from international laboratories. The main components are designed and calculated by using the Simres and NOP codes

and analytic method.The channel-cut crystal for monochromator and analyzer

and the pre-monochromator are desiged

and minimum Q of spectrometer is calculated.The results indicate that width of Darwin plat affects the minimum Q of spectrometer

the angle resolution of reflectivity planes selected in the design can ensure a Q resolution in 10

-5

order.After the monochromatic neutron guide

a vertical focus monochromator does not seem to increase neutron flux at the sample stage

to simplify the monochromator structure and its manufacturing process

we propose to use un-focus pre-monochromator after the monochromatic neutron guide.