The quasi-linear superelasticity in Ni-Mn-Ga-Co shape memory alloy displays a complete recoverable strain of similar to 3% at 473 K for the cubic phase in precursor state. The in-situ neutron diffraction experiments provide the direct evidence on the stress-induced transition from the precursor state to martensite via the continuous variation of lattice parameter and peak width under uniaxial stress field. The anomalous broadening in peak width suggests that the external stress field may trigger very high heterogeneity in lattice distortion for the initial precursor state having randomly-distributed domains with short-range strain ordering prior to the martensitic transformation. The origin of the quasi-linear superelasticity at high temperature is considered as the ruggedness of the local energy landscape. The present investigations provide new insight into the understanding of superelasticity in shape memory alloys.