The objective of this study was to increase the selectivity of hydrocarbons in bio-oil by changing the degree of closure of the reactor. Using pine sawdust with a particle size of 0.5 to 0.8 mm as the pyrolysis material and HZSM-5 with a SiO2/Al2O3 ratio of 60 as the catalyst, biomass catalytic pyrolysis experiments were carried out in a fixed bed reactor. The bio-oil compositions as a function of the closure time of the reactor and the catalyst-to-biomass weight ratios were investigated. The deactivation of the catalysts was also evaluated. The selectivity for the hydrocarbons obtained in the bio-oil was 18.99% at conditions of 550 °C reaction temperature and 20 wt% HZSM-5 catalyst usage with no closure time. When the closure time of the fixed bed reactor was 1 h, the selectivity of the hydrocarbons reached 95.28%, and the heating value reached 32.5 MJ/kg; however, the organics yield in the liquid product was only 6.2%. This investigation verified that the quality of the bio-oil could be improved significantly through the closure of the reactor, which could be due to pyrolysis vapors that could be treated by the coupling effect between a high pressure and catalytic pyrolysis.