The present invention provides a method for forming a patterned polyimide layer with the use of a positive photoresist composition. The composition comprises a cresol-type novolac resin, a diazonaphthoquinone-based sensitizer and an organic solvent; based on the cresol-type novolac resin with a total amount of 100 parts by weight, the amount of the diazonaphthoquinone-based sensitizer ranges from 40 parts to 60 parts by weight, the amount of the free cresol in the cresol-type novolac resin is lower than 2 parts by weight, and the alkaline dissolution rate (ADR) of the cresol-type novolac resin in an aqueous solution of 3.5 wt % to 7 wt % tetramethylammonium hydroxide is lower than 285 Å/s. The positive photoresist composition has excellent chemical resistance to the polyimide stripper, and can specifically improve the protective ability of the photoresist layer to the low-dielectric polyimide layer, thereby optimizing the manufacturing process and quality of the patterned polyimide layer.