The initiation and subsequent control or exploration study of chemical transformation in real time by using ultrashort laser pulses aim at femtochemistry. The real-time investigations of ultrafast dynamics of excited molecules in gas and condensed phases have attracted a great deal of attention over the last two decades. As a kind of important organic compound, aliphatic ketone is an area of much interest for many research fields, especially for atmospheric photochemistry. Via photodissociation reaction, it can release carbonyl radical whose chemical character is active and can react with hydroxyl easily. As a typical aliphatic ketone, butanone has been a research focus over the past decades. The ultrafast dissociation dynamics of butanone after excitation to the second electronically excited state (S2) with a 195.8 nm pump pulse is studied by the femtosecond pump-probe technique combined with the time-of-flight mass spectrometry (TOF-MS). Time-resolved mass spectrometry (TRMS) has proven to be a powerful technique to study the ultrafast dynamics of excited states in molecules. In this technique, the MCP detector is capable of recording time-resolved ion yield measurements of different cations by monitoring the current output directly from the anode by using an oscilloscope. This enables a time-of-flight mass spectrum to be recorded at each delay time, which is controlled by a delay stage, and the measured total signal is then integrated, yielding a time-resolved ion yield transient, which is conducted by LABVIEW software. The pump wavelength in this work is set to be 195.8 nm and the probe laser wavelength is centered at 800 nm. The complex ultrafast dynamics in butanone with 3s Rydberg state excitation and its possible decay paths and following dissociation mechanism are given. Experimental results show that the Norrish I type dissociation kinetics of butanone exhibit rich features, for it has a methyl group and an ethyl group at position. The decay time constant of the parent transient is approximately 2.23 ps0.02 ps. There is only one time constant of 2.15 ps0.02 ps for the fitting of the propionyl transient. The best fit of acetyltransient is obtained with four time constants:1=(2.400.15) ps, 2=(1.100.25) ps, 3=(0.080.02) ps, and 4=(17.720.80) ps, corresponding to S2S1 internal conversion, the primary dissociation of the S1 state generating CH3CO(), internal conversion and secondary dissociation of CH3CO() respectively. Two competitive -CC bond dissociation processes are observed and discussed. They are dissociation channels through intramolecular vibrational energy redistribution (IVR) and/or by getting over the dissociation barrier in -cleavage of butanone. But hereunder the condition of this experiment, the dissociation is the result of IVR.