The invention discloses a warning safety ladder and its installation method. The warning safety ladder includes ladder beams on two sides and several crosspieces connecting the ladder beams on two sides. The warning safety ladder is characterized in that: level meters at a 60-degree included angle with the ladder beams on two sides are disposed at the lower parts of the ladder beams on two sides; warners are arranged on the upper parts of the ladder beams on two sides and are communicated with spring push switches on an upper crosspiece at the upper part of the ladder beams on two sides, and a trough type pedal covering the upper crosspiece is disposed on the spring push switches. The warning safety ladder provided by the invention can solve the problems that correct safety angle judgment cannot be achieved during safety ladder use and early warning is impossible when a limited climbing height is exceeded, completely eradicates operation staff's use of ladders against rules, reduces security risks, and ensures the personal safety of operating staff.