My Mk2 is sagging on driver rear side, compare to passenger side, its lower about 0.5inch.
What could be wrong?
I can see the A poing in the diagram is lower on the diver side compare to passenger side. (When compare to the frame, it stick out a little more)

Do i have a uneven spring or the front spring bushing needs replace or adjust or some other thing?

 

Auriga
Jack the car, remove the spring, and replace all of the spring mounting rubbers including the one in the eyebolt.
The rubber mounting number 2 associated with your "A" is generally the main cause for a sagging rear end.
This is not difficult to do as I have done it on several MK1 and MK2 cars

It is worthwhile doing both springs because if one side is giving out you can be sure the other side will shortly follow
Cheers

 

is it possible cause by thew weight of fuel tank?

 

This is sagging!

 

Everything that Bill said. The nuts and bolts holding the brackets (3 and 4) are in a muck and bullets region and may be difficult to shift. If so, it's quicker to use (very carefully) an angle grinder. Also, be aware that the spring is heavy and the rear axle is extremely heavy. Don't let them fall on you. Also, if you completely disconnect the rear axle, it tends to fall back towards the fuel tank. Don't put your fingers between the axle and the flange on the tank.

 

Also if you can put the two springs next to each other and check if they are both still have the same curvature, if they do and the bushes look ok (abeit you should still change them) with nothing obviously wrong I would also swap the springs left to right while you have them out.

Other things to check are the structural integrity of the spring hanger box as any serious corrosion in this area could cause this (removing the rear seat base and looking at this from above tends to give a good idea of if there are any serious problems in this area) and also if the boxes have been replaced checking that they are in the correct places and are level. Also check the dampers in case one is sticking albeit this would be unlikely to be the issue.

You can alter the spring height by packing out the mounting bolts for the front mounting plate (the one with three bolts) but you won't get much of a change by doing this as the packing can only be a small amount (you would be unlikely to get 1/2").

Generaly if its like this something will be wrong and hopefully it will be fairly obvious. If its a spring make sure you change them in pairs.