Abstract: Unconventional tight reservoir has a low porosity, which is usually less than 10%, while that of shale reservoir is even less than 5%. The porosity test error of helium injection method has been considered. A mathema-tical model is established for the error of porosity determination and the statistical data of source and error limit was obtained for volume measurement. (1) The error limit of porosity determination of tight reservoir using vernier caliper is 0.59%-2.47%, while that can be reduced to 0.38%-1.04% by using a high-precision 3D laser scanner. Different from conventional reservoirs, the porosity measurement accuracy of tight reservoir should be improved and analyzed. (2) The accuracy of all porosity determination methods cannot reach the industry standard (<0.5%), so a new standard is suggested that reducing the error limit standard (i.e. <1%). The demand of volume measurement was made based on the new standard. (3) An inverse proportion is found between porosity error and core plug length, so sample length should be longer than 3 cm to reduce porosity error.