删除列表指定数值失败

[五亭桥]

num = list(range(1,21))
print(num)
for num_1 in num:
    if num_1 % 3 == 0:     #被3整除的展示出来
        print(num_1)
    else:                  #否则就从列表中删除
        num.remove(num_1)
print(num)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
3
6
9
12
15
18
[2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, 18, 20]           #隔两个才删除