Laser-induced periodic surface structures (LIPSS) were created by a 355 nm linearly polarized laser with the pulse duration of 7 ns and the pulse repetition rate of 1 Hz on polyimide films. The influences of different laser parameters on the morphology of the formed LIPSS were investigated. It is found that there is a certain fluence threshold and pulse number threshold for the generation of periodic structure. When the laser fluence is in the range of 54-586 mJ/cm², and the pulse number is between 1-50, the well-ordered LIPSS are formed with the periods between 4-6.65 μm. The pulse number and the laser fluence are changed in the experiment. With the same laser fluence, increasing the pulse number, or with the same pulse number, increasing the laser fluence, the period of the ripples can be increased. It is also observed that the formation of LIPSS is accompanied by the removal of materials, with the increase of the pulse number, the depth of the crater increases, and LIPSS can continue to appear at the bottom of the crater. In addition, in order to analyze the possible causes of the formation of periodic structure, the physical state of the material is discussed by establishing the heat conduction model when the periodic structure is formed. These works provide potential support for studying the improvement of material surface wettability, friction mechanics, and optical properties.