Your sentence is not incorrect. I don't think it breaks any rules of grammar. But it also doesn't quite sound like it comes from a native speaker. To sound like a native speaker, you need to rearrange the first three words, and there are two equally good ways of doing it. You could say: I would usually go to the park when I had free time. Or you could say: Usually I would go to the park when I had free time. However, there might also be a question about your use of "when I had" in the sentence. If you are speaking about a time in the past, then it is correct. For example, if you mean something like, "Last year when I was living in Australia, I would usually go to the park when I had free time." That would be perfectly correct. But if you are talking about something that is still true now, you would probably use "if" instead of "when". So it would be, "I would usually go to the park if I had free time." It would be quite common to hear it that way, but if you are speaking about the present, then it would be even better to make that clearer by making a bigger change in the sentence such as "I usually go to the park if I have free time." That's probably far too much information. Sorry!

I usually go to the park when i have free time!

It's not incorrect, but there are different ways of wording it which will make it sound nicer, such as: "I usually go to the park when I have free time." "I enjoy going to the park when I have free time." "When I have free, I usually go to the park."

I would usually go to the park when I had (the) free time

I usually would like to go to the park when I have time