Modern astronomy has entered a new era of all-band, multi-messenger, and space-ground synergistic observation. Optical/Infrared telescopes and their instruments have an increasing demand for high-tech innovation. Any advanced technology and process that can improve the sensitivity, extent and accuracy of detection as well as reduce the volume and cost should be concerned by the instrument design. Several ground/space-based optical surveys telescopes, such as Large Synoptic Survey Telescope, Roman Space Telescope, Euclid, and China Space Station Telescope will commission in the following five years. The upcoming massive deep sky survey data will make optical astronomy enter an unprecedented development epoch. The astronomical community hopes to build various advanced astronomical instruments to explore and investigate any objects among the different scales of the universe. The hottest research in time-domain astronomy is the identification and characteristic analysis of various transient sources, such as Gravitational-Wave Electromagnetic Counterparts, silent Black-Hole flares, and Gamma-Bursts, so it needs the spectrograph have the capability of fast response, wide-band and high efficiency with one exposure. What's more, if the instrument has multi-mode, like, multi-color imaging, spectrograph with different resolution mode, polarimetry, it will be very popular to the community. The mid-low dispersion imaging spectrometer always has the characteristics of multi-modes, high efficiency and small volume, which is suitable for the optical band following-up observation of various transient targets. This kind of instrumentation is a workhorse spectrograph for all size of telescopes around the world, such as, Low Resolution Imaging Spectrometer at Keck I telescope, Focal Reducer and Spectrograph-2 at VLT, Faint Object Camera and Spectrograph (FOCAS) at Subaru Telescope, ESO Faint Object Spectrograph and Camera at 3.6 meters telescope. However, the working band of these spectrographs is mainly visible (365~900 nm). What's more, the “FOSC” type of spectrometers was designed as a single channel, each grism needs to work with an order filter to block the spectral overlapping from other orders, and it will reduce the total efficiency. This paper takes the 1.9m optical telescope as an example, and designs a dual-channel medium-low dispersion spectrograph based on an updated “FOSC” type, which can achieve three spectral resolution modes (R=500, 2 000 and 5 500) and the working band cover from ultraviolet to near infrared (310~1 000 nm) with high efficiency. According to the grating equation, spectral resolution and other key specification, the initial parameters of the spectrograph can be derived. An approximate symmetry dual-channel design is adopted in order to make the spectrograph having a high efficiency across the whole working band and also having a compact volume. The collimator system uses a catadioptric design rather than a refractive system to improve the efficiency and then splits the spectrograph into two channels. According to the process of GRISM and VPHG, the straight-through grism efficiency is optimized by adjusting the prism material and apex angle, then 4 grism parameters are obtained for each channel. During the camera optimization, materials absorption is carefully considered, and what's more, an aspherical surface is used to reduce the total lens number and an active focusing compensation lens were chosen to meet the requirement of working at a large dynamic temperature range (-30 ℃~20 ℃). The final performance of the spectrograph is quite good, the Blue and Red channels both have excellent image quality with the maximum RMS spot radius of less than 5 μm within the full field of view in resolution mode, the peak efficiency of the spectrograph is better than 60% and the minimum efficiency at both ends of the working band is better than 20%. This paper also has a short discussion about different technologies which could be used in our design to improve the total spectral efficiency, especially the cutting-edge process of curved chips which is expected to overturn the traditional camera system design in the near future. Therefore, taking red camera as an example, the simplified camera system with a curved chip can improve the overall spectral efficiency by at least 4%, which looks very promising.