The need for developing miniature, portable and wide spectral covering spectrometers is urgent with the increase of field analysis experiment. A miniature wide-waveband monochromator which is comprised of three concave holographic gratings and two flat bedplates are designed based on the root-mean-square merit function. The three concave gratings are fixed on the upper flat bedplate symmetrically which is rotated by a stepper motor to switch the gratings and the other stepper motor is used to rotate the lower flat bedplate to realize the scanning movement. The response wavelength ranges of the three gratings are 400~1000, 1000~1700 and 1700~2500 nm with their corresponding theoretic resolution limit better than 2.5, 2.8 and 4.0 nm respectively. Manufacture errors, fixing errors and errors of the grating-switching flat bedplates are well analyzed for the three gratings. All the results show that within the error bounds the monochromator′s spectral quality can satisfy the demands of the outdoor infrared spectrum analysis very well.