A linear transmission device with capability of real-time monitoring of an amount of a lubricant includes a long shaft, a moving part, a lubricating device and a detecting module. The lubricating device includes a shell and an oil containing unit. The shell is formed with a first accommodating space. The oil containing unit is disposed in the first accommodating space and configured to provide the lubricant to an outer surface of the long shaft. The detecting module includes a temperature sensing unit and a control unit. The temperature sensing unit is configured to detect a current temperature of the oil containing unit. The control unit is connected with the temperature sensing unit and configured to: receive the current temperature; calculate a remaining amount of the lubricant of the oil containing unit based on the current temperature and an oil releasing model; and output the remaining amount.