Overcoming　the　difficulty　of　ignition　and　combustion　of　aluminium　in　water　vapour　is　the　key　to　being　applied　in　underwater　propellant　applications.　The　effect　of　coating　or　blending　20　mass%　ammonium　perchlorate　(namely,　AP@Al　or　AP-Al　samples)　on　the　ignition　and　combustion　of　Al　particles　in　a　1.0　MPa　water　vapour-Ar　environment　was　investigated　using　a　constant　temperature　pressurized　combustion　chamber　and　a　CO2　laser.　The　results　show　that　the　combustion　of　Al　particles　in　a　water　vapour-Ar　environment　is　a　non-homogeneous　reaction　and　that　agglomeration　processes　of　Al　droplets　are　observed.　The　ignition　and　combustion　properties　of　the　AP@Al　and　AP-Al　samples　under　water　vapour　were　greatly　improved　due　to　the　decomposition　of　AP　for　heat/oxygen　(particularly,　O2　provided　a　new　pathway　for　AlO(g)　generation)　supply　and　the　effect　of　dispersed　particles.　The　maximum　pressure　change　and　combustion　temperature　(Tmax)　were　increased　significantly　for　the　AP@Al　and　AP-Al　samples　combustion,　while　their　ignition　delay　time　(ti)　and　combustion　time　(tc)　decreased　remarkably.　In　addition,　the　coating　form　is　more　effective　in　the　combustion-promoting　effect　of　AP　than　the　blending　form.　Interestingly,　the　increase　in　water　vapour　concentration　is　not　conducive　to　the　ignition　and　combustion　of　samples　in　this　work.　It　is　probably　because　the　reaction　system　is　lean-fuel　where　the　excess　water　vapour　absorbs　some　of　the　heat　and　prevents　effective　collisions　among　the　radicals.　In　addition,　the　chemical　equilibrium　largely　influences　the　combustion　temperature,　resulting　in　a　conspicuous　effect　of　changes　in　water　vapour　concentration　on　the　Tmax　values.　The　reaction　interface　largely　influences　the　combustion　reaction　rate　of　Al,　so　changes　in　particle　size　have　a　significant　effect　on　the　ti　and　tc　values.