To meet the requirements of femtosecond laser micro-nanofabrication systems for high precision and wide region, the features and design specifications of infinity microscope objectives which were important components of the systems were determined. Based on the theory of primary aberration of thin lens system, the conditions to meet were concluded to correct Petzval curvature and second order spectrum for femtosecond wavelength. The objective consisted of 11 spherical lens, and all materials were glass made in China, and the use of cemented lens composed of three lens was avoided. A near infrared flat-field microscope objective, whose working wavelength was 785-815 nm, numerical aperture was 0.9, field of view in image space was 22.5 mm, magnification was 40×, was designed. Designing results show that the objective has excellent MTF, RMS wavefront errors of all fields are less than 0.08λ, and various geometrical aberrations are deeply under tolerances, the results meet the conditions of flat field and apochromatism, and energy concentration is high. Compensators are used to slacken material tolerance, manufacturing tolerances and alignment tolerances. RMS wavefront errors of all fields get less than 0.09λ after tolerances attribution and the objective can be applied actually.