NSDictionary *param = @{@"key1":@"value1",@"key2":@"value2"};
NSError *parseError = nil;
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:param options:NSJSONWritingPrettyPrinted error:&parseError];
NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
原文：http://www.cnblogs.com/damiao/p/5158176.html