Could someone tell me more about the reference value in the Fluent? I found that it is going to affect my result (x-y plot) if I change the reference value like velocity.

I suggest you have to specify you question with further details. In some cases yes, for e.g. Boussinesq approximation for bouyancy.

Temperature? Pressure? .....which reference value are you speaking of?

I just want to kow how to use the command in Report>Reference value.

when changing results from to coefficients for example pressure to Cp the reference value enered here will be used to calculate the dynamic prssure and therefore will have a large effect on the results.

these should generally reflect the freestream values if these are known.

Even for Cd computation, refernce area will affect.