A polyguluronic acid ester having a chemical name of L-polyguluronic aldonic acid-2, 3-sulfuric acid, is prepared through the steps of first obtaining L-polyguluronic aldonic acid through acid degradation and pH grading of alginic acid, proceeding secondary acid degradation and pH grading, carrying out ultrafiltration, deposition, dewatering and drying, using sulfating agent to subject the polyguluronic acid ester to sulfation, alcohol deposition, scouring, alkaline treatment, filtering, drying and disintegrating. The polyguluronic acid ester according to the invention has strong negative ion concentration property, it can be utilized in the preparation of medicament for treating or preventing urinary lithiasis.