It's fun doing sth.和It's fun to do sth.都是可以的。
It's fun后面加不定式和动名词（不是现在分词）都可以。比如《朗文实用英语搭配词典》就有这样一组划等号的同义句：
It was fun to go on the roller coaster.
=It was fun going on the roller coaster.
乘过山车很好玩。
不过，虽然 it is fun 后接不定式或动名词都可以，但若 fun 前有 no, much, great, a lot of 等修饰时，似乎后接动名词的用法比较普遍。如：
1、It’s no fun eating on your own. 一个人吃饭没意思。
2、It’s great fun sailing a boat. 扬帆驾舟十分有趣。
3、It can’t be much fun living next door to him. 与他为邻决没什么意思。
如果 fun 前面没有修饰语，则后接不定式时比较普遍。如：
1、It would be fun to have a beach party. 在海滩举行聚会会很有意思。
2、It is more fun to go with someone than to go alone. 偕伴同去比自己独自去好玩。
3、We thought it would be fun to take a ride in the boat. 我们觉得乘小船游玩会很有意思。
扩展资料：
fun的使用：
fun 英 [fʌn] 美 [fʌn]
1、n.乐趣;娱乐活动;嬉戏，嬉闹;有趣的事
2、adj.开心的;使人愉快的
3、vi.嬉闹;开玩笑
第三人称单数： funs ；现在分词： funning ；过去式： funned ；过去分词： funned。
1、fun and games (informal) 嬉戏；欢闹；寻欢作乐
activities that are not serious and that other people may disapprove of
2、make fun of sb/sth 嘲弄；取笑；拿…开玩笑
to laugh at sb/sth or make other people laugh at them, usually in an unkind way
造句：It's cruel to make fun of people who stammer. 嘲笑口吃的人是很不人道的。
3、be/become a figure of fun 是嘲笑的对象；成为笑柄
to be/become sb that other people laugh at
4、poke fun at sb/sth 拿…开心；奚落；嘲弄 同义词： ridicule
to say unkind things about sb/sth in order to make other people laugh at them
造句：Her novels poke fun at the upper class. 她的小说嘲弄上流社会。