tightly focused vectorial optical beams have found extensive applications in variety of technical fields like single-molecule detection, optical tweezers, and super-resolution optical microscopy. such applications require an accurate measurement and manipulation of focal optical fields. we have developed a compact instrument (with dimensions of 35 x 35x 30 cm(3)) to rapidly measure the intensity distribution in three dimensions of the focused fields of vectorial beams and any other incident beams. this instrument employs a fluorescent nanoparticle as a probe to scan the focal region to obtain a high spatial resolution of intensity distribution. it integrates a liquid-crystal spatial light modulator to allow for tailoring the point spread function of the optical system, making it a useful tool for multi-purpose and flexible research. the robust applicability of the instrument is verified by measuring the 3d intensity distributions of focal fields of various polarization and wavefront modulated incident beams focused by a high na (= 1.25) objective lens. the minimal data acquisition time achievable in the experiment is about 8 s for a scanning region of 3.2 x 3.2 x mu m(2) (512 x 512 pixels). the measured results are in good agreement with those predicted by the vectorial diffraction theory. published by aip publishing.