The invention discloses a safety manhole cover of a sewer. The safety manhole cover comprises a running water top cover which is provided with mutually connected gratings; the safety manhole cover is characterized by comprising a sewage draining exit which is formed by mutually connecting and surrounding the sidewall bodies of the manhole cover; a plurality of square inner grooves are formed in the sidewall bodies of the manhole cover; a plurality of lifting bracing pieces, which correspond to the inner grooves, are arranged in the inner grooves; the bracing pieces are correspondingly connected to the edge, corresponding to the sewage draining exist, of the running water top cover; an inner cavity, which is connected to the square inner groove, is arranged on each side of each of the sidewall bodies of the manhole cover; a bracing piece lifting mechanism, which is used for synchronously driving the bracing piece to lift, is arranged in each of the inner cavities; and a caution lamp tube, which is controlled by a rainwater sensor, is arranged on the running water top cover. The safety manhole cover disclosed by the invention is simple in structure and is simple and convenient to operate; the manhole cover, in the case of heavy rain, can be lifted, and by virtue of the caution lamp which is arranged on the running water top cover, the manhole cover can caution pedestrians in the case that rainwater floods the manhole cover, so as to avoid personal injury to the walking pedestrians.