Diffraction grating is the core part of spectrum instruments. To get a good performance of the instruments, different structure should be designed with different type of the grating. A spectrograph, which works in the spectrum range from 340 nm to 800 nm, was designed, whose the resolution is higher than 15nm, and the dispersion size is 28.71 mm on the focal plane. According to comparison of structure of the common spectrographs, the flat field holographic concave grating structure with many advantages was chosen. The long pass filter was used to constrain the secondary spectrum, and the optical source was also considered. As a result, the resolution of the whole system is better than 10 nm, and the size is only 190 mm×15 mm×60 mm. The flat concave-grating can be got from the market, which greatly decreases the cost of the product. Besides, only a grating can act as the spectrometer system, which can make the system easily assembly and very convenient for mass production.