Relative measurement of neutron flux is an important prospect for physical start-up. The test procedure and results of relative measurement of neutron flux based on gold foil activation method in physical start-up test of CARR (China Advanced Research Reactor) are introduced. The relative neutron flux in core and the relative neutron flux axial distribution in the irradiation channels of the heavy water tank are calculated and analyzed by using RMC Monte Carlo code. The calculated results are in good agreement with the measured values. The maximum relative error of neutron flux is less than 10%. The results also show that the neutron energy spectrum in the heavy water tank is very soft, and the measurement results of gold foil activation method can characterize the thermal neutron (e < 0.625 eV) flux distribution. But the neutron energy spectrum in the active zone of the core is relative hard, the measurement results of gold foil activation method are mainly contributed by thermal and epithermal (0.625 eV < e < 0.1 meV) neutrons, so it can not characterize the distribution of thermal neutron flux and fission power in the active region. The distribution of fission power in the active region should be measured by using 235u fission ionization chamber method.