CG-5 is new automatic land relative gravimeter developed by Canada Scintrex Company. Gravity vertical gradient can be measured by CG-5. This article mainly aims at precision analysis of the gravity vertical gradient measurement based on CG-5.This paper analyzes the error sources and error propagation formulae. In the end, it shows relation between altitude differences and observation sets in certain accuracy requirement. Results show that the precision of CG-5 vertical gradient measurement is improved when altitude difference and observation set increase. In vertical gradient measurement, 13 observation sets are needed with altitude difference at 2m to obtain the precision of ±20E.