Fast and accurate lost radioactive source positioning is of critical importance for the prevention of radiation damage in the accident. Traditional instruments with low detection efficiency cost lots pf time and energy, and may lead to harms to the operator's health. A portable detector to directly determine the incident angle of gamma rays is promising for the homeland security applications. In this work, we proposed a collimator-less 4π gamma imaging with 3-D position-sensitive detector. The distribution of photon counts is dependent on the orientation angle of the point gamma source, and the angle is reconstructed with Maximum Likelihood Expectation Maximization (MLEM) algorithm. Angular resolution of this imaging system was calculated by Cramer-Rao lower bound (CRLB). We simplified the 3-D position-sensitive detector to 2-D angular discrimination design. This design is capable of positioning the incident angle of 511 keV gamma rays. Excellent angular discrimination performance can be achieved except some particular angular positions at 0°, 90°, 180° and 270°. And the angular discrimination performance can be improved with more counts. Besides, we performed a preliminary experiment with 4 BGO crystal blocks and for all the tested angular positions ranging from 15° to 75° with an increment of 15°, the maximum error was less than 6.5°. Besides, we are now developing the positioning technology in a monolithic crystal which can be applied to the gamma imaging method. From the analytical calculation and experimental results, we conclude that the proposed 4π gamma imaging method with 3-D position-sensitive detector is effective and capable of searching for radioactive sources with excellent accuracy.