The experiment on collision between 50-keV/u Ne ⁸⁺ion and C ₃H ₄molecule is carried out by reaction microscopic imaging spectrometer. The process of forming the $\rm C_3H_4^{2+}$ divalent ion from propylene (CH ₂CCH ₂) and proacetylene (CH ₃CCH) and then dissociating to produce H ⁺and C ₃H ²⁺ $\rm C_3H_2^+$ ions and H atom is studied. Using the reaction microscope, the momentum vector of H ⁺ion and the momentum vector of $\rm C_3H_2^+$ ion are directly obtained, and then the momentum of the undetected fragment is reconstructed according to momentum conservation. By analyzing the kinetic energy of the three fragments and the total kinetic energy released from the dissociation process, the events with H atom as the third fragment are discriminated from H ⁺, and thus the H ⁺ion, $ \rm C_3H_2^+ $ ion, and H atom are identified. In addition, it is found that the sequential fragmentation pathway in which H ⁺ion and $\rm C_3H_3^+$ ion are produced in the first step followed by dissociation of $ \rm C_3H_3^+ $ into $ \rm C_3H_2^+ $ ion and H atom in the second step is the dominant dissociation mechanism according to the detailed analyses of the Dalitz plot, Newton diagram and αdistribution.