Stochastic fluctuations in a protein synthetic cascade are investigated using standard Omega-expansion technique. For the steady-state sensitivity, we show the conditions that result in the ultrasensitive "all-or-none" behavior, and for the noise propagation, we show clearly that (i) for any one given protein species in this cascade, the contributions of fluctuations in upstream proteins to its noise should be additive; and (ii) the output noise levels can vary as a function of the input concentrations and cascade length. Our results provide a possible theoretical explanation for the previous experimental studies.