感谢您的回答。我已经使用下面的代码解决了我的问题....

UITextField *txfSearchField = [self.searchbar valueForKey:@"_searchField"];
[txfSearchField setBackgroundColor:[UIColor whiteColor]];
[txfSearchField setLeftViewMode:UITextFieldViewModeNever];
[txfSearchField setRightViewMode:UITextFieldViewModeNever];
[txfSearchField setBackground:[UIImage imageNamed:@"searchbar_bgImg.png"]];
[txfSearchField setBorderStyle:UITextBorderStyleNone];
//txfSearchField.layer.borderWidth = 8.0f;
//txfSearchField.layer.cornerRadius = 10.0f;
txfSearchField.layer.borderColor = [UIColor clearColor].CGColor;
txfSearchField.clearButtonMode=UITextFieldViewModeNever;
[[UITextField appearanceWhenContainedIn:[UISearchBar class], nil] setFont:[UIFont fontWithName:@"TimesNewRomanPS-BoldItalicMT" size:20]];
[[UIBarButtonItem appearanceWhenContainedIn: [UISearchBar class], nil] setTintColor:[UIColor blackColor]];