I found an answer for my problem. it is abit long way but works for me.
I get the axis data of the figure object and redraw it as a matlab figure.
hfvt = fvtool(filterobj);
s = get(hfvt);
hchildren = s.Children;
haxes = hchildren(strcmpi(get(hchildren,'type'),'axes'));
hline = get(haxes,'children');
x = get(hline,'XData');
y = get(hline,'YData');
after this i can "plot (x,y)" and do whatever i can.