Flexible piezoresistive pressure sensors have promising applications in wearable devices, artificial intelligence, and other fields. However, developing low-cost and high-performance pressure sensors still poses a great challenge. Herein, we utilize low-cost carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) mixed in porous polydimethylsiloxane to assemble a flexible piezoresistive pressure sensor combined with interdigitated electrodes. Simultaneously, the COMSOL Multiphysics simulation analysis was performed to predict the sensing behavior of the pressure sensor, which was verified by experiments; the preparation of the pressure sensor was guided according to the prediction. Additionally, we studied the effects of the mixed conductive filler's weight ratio, the shape of the interdigital electrode, and the line width and spacing of the interdigital electrode on the performance of the sensor. Based on the interaction of the 3D porous structure and the synergistic conductive network of CB/MWCNTs, the prepared pressure sensor exhibits a high sensitivity of 3.57 kPa^-1 (∼21 kPa), a wide detection range of 0-275 kPa, fast response time (96 ms), fast recovery time (198 ms), good durability (about 3000 cycles), and good flexibility. Moreover, the fabricated sensor can monitor and recognize human activities (such as finger bending and mouse clicking), indicating that it has great potential in flexible wearable devices and other fields. It is worth noting that the preparation process of the entire pressure sensor was simple, low cost, and environmentally friendly, which provides a certain basis for industrial and commercial applications.