In order to study the mechanism of irradiation-indueed damage of botanic samples caused by low energy heavy ions, transmission energy spectrum mea-surement was performed. Kidney bean slice samples 100 µm in thickness were irradi-ated by 50 keV N⁺ ions. The irradiation beam current density was about 30 μA/cm², and the irradiation ion doses were 1×10¹⁵, 1×10¹⁶, 3×10¹⁶ and 1×10¹⁷ ions·cm^-2, respectively, A target set up that could greatly reduce the incident ion current density was designed to achieve the damage-free measurement. The 3.2 MeV H⁺ transmitted ion energy spectrum measurement was carried out before and after the irradiation. From the transmission ion energy spectrum, it was found that the kidney bean slice itself was structurally inhomogeneous compared with the PET films (C₁₀H₈O₄). Our results indicated that the average mass thickness changed little when the N+ ion dose was below 3×10¹⁶ ions·cm^-2, but changed obviously when ion dose was beyond 3×10¹⁶ions·cm^-2.