An outdoor air cleaning system includes a partition surrounding a purification space to isolate the purification space from an external space and having at least one opening connecting the purification space to the external space, an air purifying device configured to supply purified air to the purification space, and a controller configured to control the air purifying device. An openness level defined as an area ratio of the opening with respect to a surface area of the partition is about 10% or more and about 50% or less.