Spectrophotometers for colorimetry are usually composed of two independent and identical spectrometers.In order to reduce the volume of spectrophotometer for colorimetry,a design method of double-beam spectrometer is put forward.A traditional spectrometer is modified so that a new spectrometer can realize the function of double spectrometers,which is especially suitable for portable instruments.One slit is replaced by the double-slit,than two beams of spectrum can be detected.The working principle and design requirement of double-beam spectrometer are described.A spectrometer of portable spectrophotometer is designed by this method.A toroidal imaging mirror is used for the Czerny-Turner double-beam spectrometer in this paper,which can better correct astigmatism,and prevent the dual-beam spectral crosstalk.The results demonstrate that the double-beam spectrometer designed by this method meets the design specifications,with the spectral resolution less than 10 nm,the spectral length of 9.12 mm,and the volume of 57 mm×54 mm×23 mm,and without the dual-beam spectral overlap in the detector either.Comparing with a traditional spectrophotometer,the modified spectrophotometer uses a set of double-beam spectrometer instead of two sets of spectrometers,which can greatly reduce the volume.This design method can be specially applied in portable spectrophotometers,also can be widely applied in other double-beam spectrophotometers,which offers a new idea for the design of dual-beam spectrophotometers.