as we know that density of solid is constant .then why we relate ot to the volume in case of equilibrium between c(graphite) and c(diamond).

Yes you are right; density of a given solid is always constant provided its volume remains same.
If the temperature is increase or pressure in increased or the solid undergoes transition from one state to another due to which its volume is changing, its density would also change.
Density = mass/volume
Greater the volume of any solid for given mass of it, lower would be its density.