A multifunctional and modular geotechnical testing device for testing a soil sample is provided. The testing device includes a testing cell, a plurality of needle probes, and a plurality of lateral pressure-measuring sensors. The testing cell has an oedometer ring for accommodating the soil sample, with a plurality of holes located on and angularly distributed over an internal lateral surface of the oedometer ring forming a plurality of channels to access the soil sample from outside. The plurality of needle probes, each configured to house a pore water pressure-measuring sensor, is detachably mountable to the plurality of channels from outside for simultaneously measuring the pore water pressures. The plurality of lateral pressure-measuring sensors is removably attachable to an inner peripheral side of the testing cell for measuring lateral pressures of the soil sample.