Why the maxima of dB in spectrum is not 0?

I notice the vertical axis of spectrum is dB,but its maxima number is not 0.
And I notice when use dB,we should have a level that can be a reference.
So what algorithm use when dealing with dB?

Where are you looking? Are you looking at “Plot Spectrum”? Audacity Manual

I see.But I don’t understand why the maxima of dB is not 0 ?

The spectrum analysis will only hit 0dB with a pure tone. If your program material hits 0dB (in the normal time domain) all of the frequency components will sum to 0dB. (You might not see that exactly because the algorithm isn’t perfect and the information is averaged over short time-windows.)