Hi, i have a large form with a lot of fields. I used fieldset on it. How could I create a better experience to user/accessibility with this large form? I think about split it. What do you think about it?
Don't re-invent the wheel and confuse the user. Keep it simple. You could break the form down into separate parts, either into separate steps or pages and even with a progress indicator to keep the user informed of where they are.
Form design patterns:
It would be much easier to answer this if I knew a little bit more about the form and what kinds of users you want to be accessible for (ie - you would want to use different techniques if you are trying to be accessible for seniors than you would use if you want to be fully accessible for sight impaired users). Rbaker gave you a tremendous answer and I encourage you to follow his advice/read the link he provided.
However, I would like to add one thing. There is no rule that says that a form has to appear on one page. There is also no rule that says that a 'form' cannot consist of a number of individual questions. Depending on who you want to be able to use your form, a system flow like:
Do you accept our privacy policy? (Click 'yes'/'no) What is your full name? (Click 'ok') Would you like to register? (Click 'yes'/no')
You wouldn't have to refresh the page after each question, but if you gave users some idea of how close they are to being finished the form, an individual question approach might be less intimidating and more accessible for your users.
Finally, be proud of yourself for being concerned with accessibility! Best of luck!