The high-resolution numerical simulations of two-dimensional (2D) turbulent Rayleigh-Bnard convection are conducted by using the Parallel direct method of DNS (PDM-DNS) with Ra=1010 and Pr in a range from 0.05 to 20. Using the flow visualization technique, the effects of Pr on the structure of plumes and large scale circulation (LSC) are investigated. With Pr decreasing, plumes become more active and the flow turns more turbulent. When Pr4.3, pronounced LSC and corner vortex exist. The thickness of thermal boundary layer varies slightly with the value of Pr changing, which obeys a scaling law. Nusselt number (Nu) increases with Pr value increasing when Pr value is low and becomes independent when Pr value is high. Furthermore, two definitions of Reynolds number (Re) are given. The Re〈u〉 angle is calculated from the fluctuation of horizontal velocity near the center of bottom plate, and the ReUmax is calculated from maximal horizontal velocity in the mean field. Both of them follow the same scaling Re~Pr0.81.