Gaining access to the parent microstructural information is of great significance to explore transformation mechanism and mechanical properties. As a powerful route, existing automatic reconstruction techniques still have two unsolved issues for lath martensitic steels. The first one is to reconstruct annealing twin boundaries in prior austenite accurately, avoiding the distorted twin boundaries caused by orientation-shared variants on both sides. As for the second one, i.e., reconstruction of ultrafine grained prior austenite, no attempt has been succeeded, simply because the number of martensitic variants inside is insufficient for all current algorithms by now. In this paper, two novel approaches are proposed to overcome these difficulties respectively. For the first one, a three-step algorithm combined with parallel computing is developed to implement the basic reconstruction with much higher efficiency, then a physics-based method using phase field simulation is established to resolve the fake twinning morphology. The second one named as Bi-Adjacent-Block method aims at exploring morphologies (i.e., size and shape) of the ultrafine prior austenite grains (PAGs) but not intending to determine their orientations. To identify the PAG boundaries with higher accuracy, an approximate analytic determination of orientation relationship by using only two adjacent variants is developed for the first time.