i've been rejected once in applying a visa,can i still apply again?do i need to buy a ticket now?

You can submit your application again, but you'd better find out whether your application was refused first. I don't think it is a good idea to submit the same documents again.

I heard that air ticket is a necessary documents for the China tourist and business visa application. You should buy it, but do leave enough time for the visa application before your scheduled departure.

my business m was rejected because im lack of company invitation letter..so can i apply for tourist L instead?

It's possible for you to apply for a tourist L type if you have right documents. You can have a try.

can i apply the tourist visa on the next day after my business visa got rejected?

You can have a try. But I heard that after rejection, one can re-apply again after waiting for 3 months.