In the threshold of the remarkable date in the history of Zabaikalye Territory – the 310^th anniversary of Imperial Edict issued by Peter I to Nerchinsk service class Cossacks to set up monastery – the problem of preserving Assumption Day Ministry being the main religious building, which survived in extremely bad condition, is very pressing. Taking into account upcoming restoration of this unique piece of Siberian architecture of the VIII century the author summarized archival documents unrevealed before and managed to reconstruct not only the image of the lost iconostasis but the church interior in compliance with the style, and cultural and historical environment of the time when it was constructed.