The decline was discovered by union UCATT who made a Freedom of Information (FoI) request to the Health and Safety Executive (HSE).
The findings revealed there were 552 unannounced inspections in 2014/15 compared to 1,248 in 2012/2013, a fall of 55.7%.
UCATT said the drop occurred as the industry began to recover from recession and construction activity was increasing.
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Harry Frew, Regional Secretary for UCATT Scotland, said the findings were "shocking".
"It is only the prospect of an HSE inspector knocking on the door which means that many construction employers comply with safety laws," he said.
"If that deterrent doesn't exist then employers are going to be increasingly tempted to cut corners and risk workers' lives."
"The HSE needs to explain why this reduction has happened. Is this due to budget cuts or a result of specific HSE policies? Scottish construction workers deserve to be told the truth."
In addition, the decline has resulted in serious reductions in other forms of safety activity by the HSE in Scotland.
Formal enforcement action has declined by 31%, the number of formal notices issued to companies following an inspection has decreased by 25% and the number of prohibition notices has fallen by a third (33%).
(LM)
