Czerny-Turner spectrometer has been found for widespread application in the detection of Raman, fluorescence, and atmospheric remote sensing because of its simple structure and high resolution. The conventional Czerny-Turner configuration also known as unfolded C-T spectrometer, consists of two concave mirrors where the light path has no folded part. It can detect a wide range of light, so it is very suitable for dual-band detection. Dual-band spectral detection is mostly used for dual-channel spectral detection systems in the visible infrared band, dual-wavelength-excitation particle fluorescence spectrometer. However, for dual-band or multi-band detection, the spectrometer based on the Czerny-Turner structure often needs to be deformed, and the detector is always selected as area array detectors and detectors even need more than one. It makes the structure of the system more complex and increases the cost. Considering that linear array detectors are more beneficial than planar array detectors for the detection of one-dimensional spectral information, and linear array photomultiplier tube has higher detection efficiency for weak signals and has been verified for effectiveness in the dual-laser-induced fluorescence detection system. We proposed a modified optical design of a combined unfolded Czerny-Turner Spectrometer sharing one common linear-array detector. In the proposed design, the astigmatism of the system is controlled by using divergent illumination on the grating without introducing any additional optical elements, which is used for the correction of astigmatism in the Czerny-Turner spectrometer. By considering the condition of sharing one detector and the astigmatism correction method of the illuminating grating, the proposed design method of the system is provided, and the proposed design method of the system is provided. Zemax is used for simulation and optimization of the proposed optical system model. The spectrometer can detect two bands of light i.e., 280~460 nm and 380~560 nm. We have also analyzed the reciprocal linear dispersion, which demonstrates that the reciprocal linear dispersion at the central wavelength of two wavebands are 6.03 nm/mm,6.10 nm/mm, respectively. That means the proposed system satisfies the spectral resolution of 6 nm, and the spectral resolution difference of the two spectrometers in a whole working band is less than 0.6 nm. Finally, the RMS spot size of the system is also analyzed theoretically. In the first band (i.e., 280~460 nm) the maximum RMS spot is 67.1 μm at 280 nm, while in the second band (i.e., 380~560 nm) the maximum RMS spot is 53.2 μm at 560 nm. The 80% range of RMS spot radius is less than 50 μm in these two bands. In view of the design and simulation results, the prototype model is developed and the experimental verification finished. Mercury lamp was used for the optical inlet of spectrometer as a light source, and the experimental results were obtained by 32 linear array photomultiplier tube, the results demonstrate that the positions of several spectrum peaks detected are consistent with the simulation. This proposed method extends the application of the system structure with two light paths and one common linear detector in the elaborate and expensive designs, which may be beneficial for the non-imaging design of dual-waveband spectral systems.