Abstract: The double complex peptide composed of proline (Proline, P) and arginine (Arginine, R) is a toxicity-rich polypeptide produced by the abnormal amplification of C9ORF72 gene in the human body. The abnormal amplification and liquid-liquid phase separation (LLPS) behavior of the peptide are important causes of neurodegenerative diseases, so to study the regulatory behavior of the peptide is helpful to the therapeutic treatment of related diseases. In the paper, optical microscopic imaging and ultraviolet spectrophotometric technique were used to study the LLPS phenomenon and the regulation of related 35 proline-arginine repeat peptides (PR₃₅), and the regulation mechanism was analyzed. It was found that the occurrence of LLPS in PR₃₅ depended on the concentration of potassium chloride in the solution. LLPS did not take place when the concentration of KCl was lower than 2 700 mmol/L, but LLPS took place when the concentration was higher than 2 700 mmol/L. In this critical concentration (2 700 mmol/L) of the KCl solution, after adding different mass fractions of the molecular crowding agent polyethylene glycol (PEG1000), it was found that PEG had a promoting effect on LLPS, and the promoting effect increased with the increase of mass fraction of PEG concentration. At the same time, the conclusion of ultraviolet spectrophotometric absorbance analysis was consistent with the results of microscopic survey.