[kamiyu]

I do not want why that when I am doing UV-vis absorption measurement, the absorbance below 300nm fluctuate strongly: some point Absorbance 4 but suddenly drops to -1.2 something. The spectrum is a mess in the region below 300nm

Other wavelengths are OK.

Anyone has experience???

[mjc123]

When the absorbance is that high, almost no light is being transmitted through your sample. Since absorbance = log(I₀/I_t), where I₀ and I_t are the incident and transmitted intensities respectively, small random fluctuations in the transmitted signal (due to e.g. stray light or instrumental noise) will give rise to large variations in the apparent absorbance. To measure absorbance accurately in this region you will need to dilute your sample - I suggest by at least a factor of 100, if you are getting an absorbance of 4. (And if you're using a 10 mm cell, try using a 1 mm cell.)

[Babcock_Hall]

One, are you using the deuterium lamp, as opposed to the tungsten lamp?  Two glass and most plastic cuvettes are unsuitable for work in the UV region.  Some plastic cells can be used at 280 nm, but quartz cells are certain to work.  Either of these issues can be checked by running the solvent without the chromophore.  It would help if we knew the chromophore.