The peptides were precipitated after further dissolved in 1xPBS to 0.3 mM from the stock of 10 mM in DMSO. May I know what I should do with these peptides?

According your situation, it seemed it was the high concentration of PBS caused the precipitation. We recommend to add water directly. The general rule of diluting stock solution is to add water (instead of PBS buffer) first, making sure the stock solution do not precipitate in water, then add 10X PBS buffer. The charge in PBS buffer causes precipitation sometimes.