Objective To observe the changes in blood resistivity of simulated weightlessness rat by electrical impedance method and to discuss its mechanism. Methods Experimental animals were divided into normal group and simulated weightlessness group. Simulated weightlessness was achieved with the method of tail-suspension. The impedance spectroscopy of blood was measured with Agilent 4294A impedance analyzer over the frequency of 0.01-100MHz, and 80 frequency points in this frequency range were selected, each frequency point automatically scan measured three times to take the mean(AC excitation signal source voltage:  0.5V). The simulated weightlessness on blood resistivity spectral characteristics was observed by the Bode diagram, the Nyquist diagram and the Nichols diagram. Results The resistivity of the simulated weightlessness rat blood decreased, mainly in the low-frequency limiting value of the complex resistivity amplitude |ρ*|0, the high-frequency limiting value of complex resistivity amplitude |ρ*|∞, complex resistivity amplitude increment (Δ| ρ*|=|ρ*|0-|ρ*|∞), the amplitude of phase angle peak θp, the low-frequency limiting value of the real part of complex resistivity ρ′0, the peak amplitude of the imaginary part of complex resistivity ρ"p, and the amplitude logarithmic of complex resistivity at the low-frequency limiting lg|ρ*|0 all decreased compared with those of the control group. The 1st and 2nd characteristic frequencies of blood (fC1 and fC2) increased respectively compared with the control group. Conclusions Simulated weightlessness might decrease the resistivity and increase the conductivity of the plasma, red cell and hemoglobin.