I sold nifty option intraday using MIS. I found that for 1 lot of nifty option 8000 pe(OTM),Rs. 28000 nearly got blocked. Why the margin requirement so high in intraday?

Try Bracket Order.

Writing/selling an option is a risky thing, wherein writers look for limited (fixed) gains, in the form of premium that they receive from the option buyers, for unlimited potential losses. Due to this potential risk, the exchange blocks the margin. Generally, this amount equals the margin required for buying/selling futures. For MIS Option Selling or MIS Futures buying/selling, the margins are significantly lower than the margin required for overnight position. Because the risk is lower in the former case.

what benefit will i get by using the bracket order? i have never used it. Do i need less margin in intraday trade by using bracket order?

lesser margin required for intra. lot of info provided in this site n zerodha site.