"The inflow of migrants over the last three days has exceeded all manageable possibilities," a statement from the Slovenian government said, adding that parliament would be asked to approve legislation allowing soldiers to help in the crisis "under very specific circumstances".
Under current law, the army can only provide technical and logistical support.
However, "this does not mean a state of emergency," Prime Minister Miro Cerar told journalists.
State radio said parliament would debate the emergency legislation on Tuesday and that it could enter into force within days.
More than 8,000 people streamed into the tiny EU member state from Croatia on Monday, with Ljubljana warning this largely exceeded its daily quota.
The nation of two million people has become a new key transit point on the migrant trail, after Hungary sealed its Croatian border with a razor-wire fence to migrants on Saturday just weeks after it had already shut its Serbian frontier.
Tens of thousands -- many fleeing violence in Syria, Africa and Afghanistan -- have been making their way from Turkey to the Balkans in recent months, hoping to reach Germany, Sweden and other EU states.
Slovenia criticised Zagreb for lifting border restrictions at Croatia's frontier with Serbia on Monday night, allowing the migrants who had been stranded in wet and muddy conditions for hours to trek to Slovenia.
Ljubljana called for greater European solidarity, warning it was "delusional" to expect small individual countries to handle the spiralling humanitarian crisis alone.
"Slovenia calls on the European Union states and institutions to engage actively in dealing with this disproportionate weight for our state... European solidarity is being challenged." the government statement said.
"It is delusional to expect a country of two million to (accomplish) what much larger countries haven't been able to."
Ljubljana has also accused neighbouring Austria of capping its intake of migrants and only allowing in 2,000 per day, a claim Vienna rejected. "There are no restrictions in place," interior ministry spokesman Karl-Heinz Grundböck told AFP on Monday.