We report the gain factor, detection limit, and linearity of a femtosecond time-resolved spectrometer for ultraweak fluorescence based on a $400\mathrm{nm}$ pumped femtosecond noncollinear optical parametric amplifier. By employing the parametric fluorescence amplifier, the time-resolved fluorescence spectra of Rhodamine 6G dye in ethanol was measured. It is proved that a good spectral fidelity has been achieved in the measurement of fluorescence spectra with this technique. The measured gain factor of this parametric fluorescence amplifier is found to be $\sim 1.2\times {10}^6$ , and the detection limit is $5.1\mathrm{aJ}$ per pulse, corresponding to 15 fluorescence photons at $580\mathrm{nm}$ . With this technique, we have acquired the transient fluorescence spectra as well as the fluorescence decay profile of $\mathrm{Cd}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ $(x=0.78)$ nanobelts. Our results demonstrate the feasibility of this technique as an ultrasensitive time-resolved fluorescence spectrometer.