你的主要问题是你错过了 DELIMITER 在存储过程开始之前。
此外，还有 IF 过程末尾的语句格式不正确：语法为 IF ... THEN ... END IF; . 最后，检查是否为空 IS NULL 而不是 = NULL .
编译成功：

SET @listOfStates = 'AK, AL, AR, AS, AZ, CA, CO, CT, DC, DE, FL, GA, GU, HI, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MP, MS, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, OR, PA, PR, RI, SC, SD, TN, TX, UM, UT, VA, VI, VT, WA, WI, WV, WY';

SET @item = SUBSTRING_INDEX(@listOfStates, ',', 1); 
SET @itemValue = 0;

DELIMITER //

CREATE PROCEDURE populate()
BEGIN
    SET @strLen = LENGTH(@listOfStates); 

    label1: LOOP

        SET @strLen = LENGTH(@listOfStates);
        SET @item = SUBSTRING_INDEX(@listOfStates, ',', 1); 
        SELECT COUNT(*) INTO @itemValue FROM us_accidents u WHERE u.state LIKE item;

        INSERT INTO state_cases (name, cases) VALUES (@item, @itemValue);

        SET @SubStrLen = LENGHT(SUBSTRING_INDEX(listOfStates, ',', 1));
        SET @listOfStates =  MID(listOfStates, SubStrLen, strLen);

        IF listOfStates IS NULL THEN
            LEAVE label1;
        END IF;

        ITERATE label1;

    END LOOP label1;

END; 
//

DELIMITER ;