How to negotiate salary? Please help!!

They want me to go to work ASAP, but salary is 15% lower than what I asked for. HR told me they usually give 20% raise to candidates based on their current one. But they offered me 25%, it's the best raise they offered. But I don't care, my base salary is very low, at least 20% lower than the market. That's the main reason I want to change job. I never did negotiation before, could someone please give me some advices? Thank you very much, they are expecting my response now......

Thanks....