According to the performance and application of the hyperspectral imager with wide field of view and large relative aperture, a simple two-mirror fore objective with biased field of view consisting of one convex and one concave mirror is studied and designed. Based on its Gaussian optics and astigmatism analysis with Young′ s formulas and under the normalization condition with respect to the optical system′s focal length，the vertex curvature radius of the convex mirror should be chosen within the interval of [2.5 mm, 3.24 mm). The approach to determine its initial structural parameters which meet with expected performance and the corresponding results are presented. As an example, a fore two-mirror imagery telecentric objective with biased field of view for nonobscuration is optimally designed. Its working wavelength ranges from 0.4 to 1.0 mm, and its relative aperture and its linear field of view is respectively up to 1/1.8 and 40°. Both of mirrors are oblate ellipsoid surface with only conic coefficient. The designed lens has many advantages of simple and compact construction，near diffraction-limited imaging quality，imagery telecentricity, high speed and collection power，and wide and flat field of view. It will become the ideal fore objective of imaging spectrometers with high and spectral resolution and high signal-to-noise.