Abstract: The present invention relates to a novel process for preparing polyimide, which comprises: reacting a di-anhydride with diamine to form polyamic acid, applying or casting the polyamic acid on a substrate, evaporating the solvent contain therein to control the volatile content in the resulting polyamic acid at not more than 10 wt %, rewinding the polyamic acid-coated substrate at a tensile strength of not more than 20 kgf/cm2, transferring into an oven, heating the coated substrate at a gradient temperature to cyclize the polyamic acid into polyamide, to obtain a flexible laminate having excellent appearance in good yield. According to the present process, a polyimide flexible film having excellent properties can be produced in a low cost and a high yield.