To extend the working spectral range of spectrometer while keeping the resolution the same, a freeform lens was designed based on non-imaging principle, which could collimate the broadband source light into two beams with different wavelengths in different direction, to achieve the spectrum-fold on a two demention array detector. First of all, a first-order partial differential equations set was obtained according to the vector relationship between the incident rays and the refractive rays on the freeform surface. Then Runge-Kutta methods was adopted to get the numerical solution of the partial differential equations which are the discrete points of the freeform surface, and the freeform lens was constructed. The two beam collimating of the source was simulated in ZEMAX software, and the irradiance maps of the two collimated beams were simulated in Tracepro software. Finally, the freeform lens was appllied to a spectrometer to collimate a source with wavelength ranging from 800 nm to 2 400 nm into two beams with wavelength from 800 nm to 1 600 nm and from 1 600 nm to 2 400 nm in different direction. After being dispersed by one grating and focused by imaging lens, two paralle and end to end spectra were distributed in the detector, and the spectral resolution was better than 10nm. The simulation results show that, by using this kind of freeform lens, both high resolution and wide working wavelength range can be achieved in one spectrometer, and the structure of the spectrometer becomes much more compact.