Fermi National Accelerator Laboratory,Batavia, Illinois 60510, USA(Dated: April 30, 2013)The development of the microbunching instability is studied for the linac of the proposed ShanghaiSoft X-ray Free-Electron-Laser facility (SXFEL) by analytic formulae as well as numerical simula-tions with the aid of two different codes. The process is investigated in detail and the growth rates(gains) of the instability under various conditions are compared. The results indicate that the limi-tations from numerical computations in the present simulation model must be taken into account.Moreover, the appearance of higher-order mode excitations in the simulations suggests that furtherimprovement of the current theory is necessary. A mechanism of introducing shot noise into beamprofile as the beam passes through a chicane is proposed. The key issues that drive the instabilityare analyzed.