The formation tester while drilling can provide information on formation pressure and mobility, and is an important logging while drilling (LWD) tool in oil and gas reservoir exploration. The circuit, software, and mobility algorithm are the core components of the measurement and control system of the While Drilling Formation Tester (FTWD). IFPT800 is a brand new FTWD tool. This paper innovatively designs a very simple and powerful master–slave circuit scheme with a compact installation method for IFPT800. All circuits can withstand working environments up to 150 ℃ for a long time, and thick film integrated circuits can reach 175 ℃. This paper uses the limited resources of the MSP430 processor to develop an automatic execution software for formation pressure measurement and flow plug removal (FPR) based on state machines. This paper designs an down-hole real-time algorithm for calculating formation mobility, which is the first to implement integrated formation mobility calculation on a 16 bits microprocessor, greatly improving the efficiency of LWD tool operation. The impact of noise in formation pressure measurement and formation mobility calculation is analyzed, and the upper limit of noise in mobility calculation is derived. The innovative design of IFPT800 includes FPR and real-time calculation of down-hole formation mobility. At the end of this paper, case studies of formation pressure testing and FPR were presented, in which the design of this article achieved good results. IFPT800 is particularly suitable for formation pressure testing and formation mobility calculation in low-permeability reservoirs. It has been operating in tens of wells in the Bohai Sea region and has achieved good results.