The invention discloses low-sugar-content purple-sweet-potato chocolate and a preparation method thereof. The low-sugar-content purple-sweet-potato chocolate is prepared from the following raw materials in parts by weight: 30-60 parts of cocoa butter, 10-40 parts of cocoa powder, 10-30 parts of milk powder, and 0.1-4 parts of edible essences, 8-14 parts of dried purple sweet potato fragments, 0.01-0.05 part of soy lecithin, and 1-15 parts of a fructus momordicae extract. The fructus momordicae extract is utilized as a sweetener instead of granulated sugar in the purple-sweet-potato chocolate, so that the calories of the chocolate are effectively reduced; thus, the prepared chocolate is more suitable for people with sugar intake restriction to eat. Moreover, the added dried purple sweet potato fragments are capable of adding crispy taste to the chocolate, so that the chocolate has both soft and crispy taste; thus, the pleasure of eating is increased.